serial: liteuart: rx loop should only ack rx events

While receiving characters, it is necessary to acknowledge each one
by writing to the EV_PENDING register's EV_RX bit. Ensure we do not
also gratuitously set the EV_TX bit in the process.

Signed-off-by: Gabriel Somlo <gsomlo@gmail.com>
Reviewed-by: Jiri Slaby <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20221123130500.1030189-8-gsomlo@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>