tty: serial: remove set but not used variable 'error'

Fixes gcc '-Wunused-but-set-variable' warning:

drivers/tty/serial/pmac_zilog.c: In function 'pmz_receive_chars':
drivers/tty/serial/pmac_zilog.c:222:30: warning:
 variable 'error' set but not used [-Wunused-but-set-variable]

Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>