tty: omap-serial: remove set but unused variable

tty: omap-serial: remove set but unused variable

Fix the following warning:
drivers/tty/serial/omap-serial.c: In function serial_omap_rlsi:
drivers/tty/serial/omap-serial.c:496:16: warning: variable ch set but not used [-Wunused-but-set-variable]

The character read is useless according to the table 23-246 of the omap4
TRM. So we can drop it.

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Xiongfeng Wang <wangxiongfeng2@huawei.com>
Link: https://lore.kernel.org/r/1575617863-32484-1-git-send-email-wangxiongfeng2@huawei.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>