Fix undefined `cs` variable

Attempting to compile without this fix results in this error
```
main.c:226:9: error: 'cs' undeclared (first use in this function)
  226 |         cs = readbyte_blocking();
      |         ^~
```
This fix declares `cs` as a `uint8_t`. Tested and was able to read/write to a MX25L6406E and MX25L3206E

main.c

@@ -223,7 +223,7 @@ void s_cmd_s_pin_state() {
 }
 
 void s_cmd_s_spi_cs() {
-	cs = readbyte_blocking();
+	uint8_t cs = readbyte_blocking();
 	if (cs >= NUM_CS_AVAILABLE)
 		sendbyte_blocking(S_NAK);
 	return;