serial: st-asc: Potential error pointer dereference

It looks like we intended to return an error code here, because we
dereference "ascport->pinctrl" on the next lines.

Fixes: 6929cb00 ("serial: st-asc: Read in all Pinctrl states")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Lee Jones <lee.jones@linaro.org>
Cc: stable <stable@vger.kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>