tty: serial: mpc5xxx: fix error handing in mpc52xx_uart_init()

Add the missing uart_unregister_driver() and uninit before return
from mpc52xx_uart_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>