tty: serial: qcom-geni-serial: refactor qcom_geni_serial_send_chunk_fifo()

Shuffle the code a bit, drop unneeded variables, make types of others
more consistent and use uart_xmit_advance() instead of handling
tail->xmit manually.

Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>
Link: https://lore.kernel.org/r/20221229155030.418800-11-brgl@bgdev.pl
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>