igmp: mask the port number before shifting by it

The RTL tag carries a 4-bit port number for received frames, and the two
places that turn it into a mask bit take the whole byte:

	entry.pmask |= (1L << (IGMP_I->rtl_tag.pmask >> 8));

The comment beside it already says only the low four bits count, but nothing
takes them. rtl837x_stp.c:424 does the same extraction and does mask:

	stp_scratch = ((uint8_t)HTONS(STP_I->rtl_tag.pmask)) & 0x0f;

With a count above 15 the result is not a bit at all. SDCC narrows the 1L to
16 bits and emits the shift as a djnz loop, so a count of 200 spins the loop
200 times and leaves zero behind: the join adds no port and the leave clears
none, neither saying anything. It also spends those iterations inside packet
handling.

Mask to four bits and shift a uint16_t, which is the width the surrounding
code works in. Six bytes of BANK1.

Worth saying that this does not make snooping work on its own. struct
igmp_pkt has no vlan_tag field while struct eth_in describes the same frame
with one, so every IP and IGMP field below rtl_tag is read four bytes early.

rtl837x_igmp.c

@@ -295,7 +295,7 @@ void igmp_packet_handler(void) __banked
 			entry.pmask |= ((uint16_t)sfr_data[3]) << 2;
 		}
 		// Update (found) entry with portmask from trapped Packet
-		entry.pmask |= (1L << (IGMP_I->rtl_tag.pmask >> 8));  // Swap bytes from network order, only 4 LSB count
+		entry.pmask |= ((uint16_t)1) << ((IGMP_I->rtl_tag.pmask >> 8) & 0x0f);  // Swap bytes from network order, only 4 LSB count
 //		print_string("\nPort-Mask: "); print_short(entry.pmask); write_char('\n');
 	} else if (IGMP_I->igmp_rtype == 0x3){  // Leave group
 		if (sfr_data[2] & 0x10) {
@@ -312,7 +312,7 @@ void igmp_packet_handler(void) __banked
 			write_char('\n');
 #endif
 			// Remove portmask of IGMP packet from entry
-			entry.pmask &= ~(1L << (IGMP_I->rtl_tag.pmask >> 8));  // Swap bytes from network order, only 4 LSB count
+			entry.pmask &= ~(((uint16_t)1) << ((IGMP_I->rtl_tag.pmask >> 8) & 0x0f));  // Swap bytes from network order, only 4 LSB count
 //			print_string("\nPort-Mask: "); print_short(entry.pmask); write_char('\n');
 		} else {
 			print_string("IGMP Entry already deleted\n");