tty: serial: 8250: serial_cs: Remove unused/unchecked variable 'err'

tty: serial: 8250: serial_cs: Remove unused/unchecked variable 'err'

Fixes the following W=1 kernel build warning(s):

 drivers/tty/serial/8250/serial_cs.c: In function ‘multi_config’:
 drivers/tty/serial/8250/serial_cs.c:562:7: warning: variable ‘err’ set but not used [-Wunused-but-set-variable]

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jirislaby@kernel.org>
Cc: Colin Ian King <colin.king@canonical.com>
Cc: "David A. Hinds" <dahinds@users.sourceforge.net>
Cc: linux-serial@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20201104193549.4026187-15-lee.jones@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>