gnss: serial: fix synchronous write timeout

Passing a timeout of zero to the synchronous serdev_device_write()
helper does currently not imply to wait forever (unlike passing zero to
serdev_device_wait_until_sent()). Instead, if there's insufficient
room in the write buffer, we'd end up with an incomplete write.

Fixes: 37768b05 ("gnss: add generic serial driver")
Cc: stable <stable@vger.kernel.org>     # 4.19
Signed-off-by: Johan Hovold <johan@kernel.org>