gps: fix Septentrino serial read (#22936)

For Septententrino we seem to sometimes fill the buffer pretty full.

If we ask for too much, readAtLeast will fail completely and make the
GPS discovery logic fall over. Therefore, let's not ask for too much and
just read what we can given the available buffer.

Signed-off-by: Julian Oes <julian@oes.ch>

src/drivers/gps/gps.cpp

@@ -471,14 +471,14 @@ int GPS::callback(GPSCallbackType type, void *data1, int data2, void *user)
 int GPS::pollOrRead(uint8_t *buf, size_t buf_length, int timeout)
 {
 	int ret = 0;
-	const unsigned character_count = 32; // minimum bytes that we want to read
+	const size_t character_count = 32; // minimum bytes that we want to read
 	const int max_timeout = 50;
 	int timeout_adjusted = math::min(max_timeout, timeout);
 
 	handleInjectDataTopic();
 
 	if (_interface == GPSHelper::Interface::UART) {
-		ret = _uart.readAtLeast(buf, buf_length, character_count, timeout_adjusted);
+		ret = _uart.readAtLeast(buf, buf_length, math::min(character_count, buf_length), timeout_adjusted);
 
 // SPI is only supported on LInux
 #if defined(__PX4_LINUX)
@@ -1560,4 +1560,4 @@ int
 gps_main(int argc, char *argv[])
 {
 	return GPS::main(argc, argv);
-}
+}