serial: mcf: missing uart_unregister_driver() on error in mcf_init()

Add the missing uart_unregister_driver() before return
from mcf_init() in the error handling case.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>