Fix warnings seen on SDCC 4.6.0 due to missing const qualifiers

Building with SDCC 4.6.0 emits tons of warnings e.g.:

	cmd_parser.c:31: warning 356: object in read-only code space should be const
	cmd_parser.c:36: warning 357: pointer to object in read-only code space should be pointer to const

These of course become fatal when building with --Werror.

Comparing resultant binaries, it seems that a single string is shifted
(deduplication?) but the numbers emitted at the end of the build are
otherwise identical.

sfp.c

@@ -35,12 +35,12 @@ __code enum sfp_quirk {
 };
 
 struct sfp_quirk_entry {
-	__code char *vendor; // Set vendor or model to 0 to act as wildcard
-	__code char *model;
+	__code const char *vendor; // Set vendor or model to 0 to act as wildcard
+	__code const char *model;
 	uint8_t quirks;
 };
 
-static __code struct sfp_quirk_entry sfp_quirk_table[] = {
+static __code const struct sfp_quirk_entry sfp_quirk_table[] = {
 	{ "QSFPTEK", "QT-SFP+-T", SFP_QUIRK_DDM },
 };