refactor: use uint8_t for serial buffers

Match crsf.h's use of fixed-width types for byte buffers
instead of unsigned char.

celrs/serial.c

@@ -37,7 +37,7 @@ void cel_serial_close(cel_serial_port* port) {
     free(port);
 }
 
-size_t cel_serial_read(cel_serial_port* port, unsigned char* buf, size_t len, int timeout_ms) {
+size_t cel_serial_read(cel_serial_port* port, uint8_t* buf, size_t len, int timeout_ms) {
     (void)port;
     (void)buf;
     (void)len;
@@ -46,7 +46,7 @@ size_t cel_serial_read(cel_serial_port* port, unsigned char* buf, size_t len, in
     return 0;
 }
 
-size_t cel_serial_write(cel_serial_port* port, unsigned char const* buf, size_t len) {
+size_t cel_serial_write(cel_serial_port* port, uint8_t const* buf, size_t len) {
     (void)port;
     (void)buf;
     (void)len;

celrs/serial.h

@@ -1,5 +1,6 @@
 #pragma once
 #include <stddef.h>
+#include <stdint.h>
 
 /* Opaque serial port handle */
 typedef struct cel_serial_port cel_serial_port;
@@ -15,10 +16,10 @@ cel_serial_port* cel_serial_open(char const* path, int baud_rate);
 void cel_serial_close(cel_serial_port* port);
 
 /* Read up to len bytes. Returns bytes read, 0 on timeout/error. */
-size_t cel_serial_read(cel_serial_port* port, unsigned char* buf, size_t len, int timeout_ms);
+size_t cel_serial_read(cel_serial_port* port, uint8_t* buf, size_t len, int timeout_ms);
 
 /* Write data. Returns bytes written, 0 on error. */
-size_t cel_serial_write(cel_serial_port* port, unsigned char const* buf, size_t len);
+size_t cel_serial_write(cel_serial_port* port, uint8_t const* buf, size_t len);
 
 /* Flush output buffer */
 void cel_serial_flush(cel_serial_port* port);