tty: ipwireless: Fix potential NULL pointer dereference

There is a potential NULL pointer dereference in case
alloc_ctrl_packet() fails and returns NULL.

Fixes: 099dc4fb ("ipwireless: driver for PC Card 3G/UMTS modem")
Signed-off-by: YueHaibing <yuehaibing@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>