tty: serial: qcom_geni_serial: Remove xfer_mode variable

The driver only supports FIFO mode so setting and checking this variable
is unnecessary. If DMA support is ever added then such checks can be
introduced.

Signed-off-by: Ryan Case <ryandcase@chromium.org>
Reviewed-by: Evan Green <evgreen@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>