rtl8372_init: set SDS 0 to SGMII as the stock firmware does

The masks were 0, so 0x3e2 was ORed into the reset value 0xbff and both
SerDes stayed off on boards without 10G ports (n_10g == 0), e.g. the
HC-SWTG124AS. With SDS 0 off RTL837X_REG_SEC_COUNTER does not hold a
written value: it reads 0, so NTP never retried after a failed first
attempt at boot and froze the clock after a sync.

Tested with regset on an HC-SWTG124AS: the register keeps a write with
SDS 0 in SGMII or 10GR, not with SDS 0 off (also not with only SDS 1
on).

rtl837x_init.c

@@ -221,9 +221,11 @@ void rtl8372_init(void) __banked
 	} else if(machine.n_10g == 2) {
 		REG_SET(RTL837X_REG_SDS_MODES, 0x1ad); // Both 10g ports use SDS_QXGMII
 	} else {
+		// Replace the reset value 0xbff (both SDS off) instead of ORing into it:
+		// RTL837X_REG_SEC_COUNTER only holds a written value while SDS 0 is on
 		reg_read_m(RTL837X_REG_SDS_MODES);
-		sfr_mask_data(1, 0, 0x03);
+		sfr_mask_data(1, 0xff, 0x03);
-		sfr_mask_data(0, 0, 0xe2);
+		sfr_mask_data(0, 0xff, 0xe2);
 		reg_write_m(RTL837X_REG_SDS_MODES);
 	}