tty: serial: qcom_geni_serial: Remove set_rfr_wm() and related variables

The variables of tx_wm and rx_wm were set to the same define value in
all cases, never updated, and the define was sometimes used
interchangably. Remove the variables/function and use the fixed value.

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