isletter(): optimized version

Less instructions and jumps.
Saves 39 bytes.

```patch
1634,1651c1621,1628
<       mov     r7,a
<       mov     r6,a
< ;     cmd_parser.c:63: return (l >= 'a' && l <= 'z') || (l >= 'A' && l <= 'Z');
<       cjne    r7,#0x61,00326$
< 00326$:
<       mov     _parse_vlan_sloc0_1_0,c
<       jc      00153$
<       mov     a,r7
<       add     a,#0xff - 0x7a
<       mov     _parse_vlan_sloc0_1_0,c
<       jnc     00149$
< 00153$:
<       cjne    r6,#0x41,00329$
< 00329$:
<       mov     _parse_vlan_sloc0_1_0,c
<       jc      00132$
<       mov     a,r6
<       add     a,#0xff - 0x5a
---
> ;     cmd_parser.c:66: l |= 0x20;
> ;     cmd_parser.c:67: l -= 'A';
>       orl     a,#0x20
> ;     cmd_parser.c:68: return (l <= ('Z'-'A'));
>       add     a,#0xbf
>       add     a,#0xff - 0x19
>       cpl     c
> ;     cmd_parser.c:245: if (cmd_words_b[2] > 0 && isletter(cmd_buffer[cmd_words_b[2]])) {
1653,1654c1630
<       jc      00132$
< 00149$:
---
>       jnc     00132$
```

cmd_parser.c

@@ -60,7 +60,12 @@ __code uint8_t phys_to_log_port[6] = {
 
 inline uint8_t isletter(uint8_t l)
 {
-	return (l >= 'a' && l <= 'z') || (l >= 'A' && l <= 'Z');
+	// return (l >= 'a' && l <= 'z') || (l >= 'A' && l <= 'Z');
+
+	// Make it upper case
+	l |= 0x20;
+	l -= 'A';
+	return (l <= ('Z'-'A'));
 }