Use correct datatype for return value of FspTimer::get_available_timer()

[mast-eu]

This is a follow-up for #282 :

I missed in my previous PR that the return value from FspTimer::get_available_timer() is actually an int8, but in Arduino_LED_Matrix.h is stored as uint8.
Therefore the following if (added in #282) cannot work as intended:

ArduinoCore-renesas/libraries/Arduino_LED_Matrix/src/Arduino_LED_Matrix.h

Lines 175 to 178 in 2c8dd57

	uint8_t ch = FspTimer::get_available_timer(type);
	if(ch == -1) {
	return false;
	}

This PR is to use the correct datatype. Ideally it should have been included in my previous PR.