pch_uart: remove set but not used variable 'tx_empty'

tx_empty is not used after setting its value. It is safe to remove
the unused variable.

Signed-off-by: zhong jiang <zhongjiang@huawei.com>
Reviewed-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>