fix(serial): Make length parameter a size_t

Make the length parameter a `size_t` instead of a `intptr_t`.  A signed
value does not make any sense here and just leads to weird behavior if a
negative value is given nonetheless.

See issue card10/firmware#192

Signed-off-by: Rahix <rahix@rahix.de>

epicardium/epicardium.h

@@ -320,7 +320,7 @@ API(API_THERMISTOR_VOLTAGE, int epic_read_thermistor_voltage(float *result));
  * :param length:  Amount of bytes to print.
  */
 API(API_UART_WRITE_STR, void epic_uart_write_str(
-	const char *str, intptr_t length
+	const char *str, size_t length
 ));
 
 /**

epicardium/modules/serial.c

@@ -54,7 +54,7 @@ void serial_return_to_synchronous()
 /*
  * API-call to write a string.  Output goes to both CDCACM and UART
  */
-void epic_uart_write_str(const char *str, intptr_t length)
+void epic_uart_write_str(const char *str, size_t length)
 {
 	if (length == 0) {
 		return;