wireless: ipw2100: check result of kzalloc()

If kzalloc() fails return with -ENOMEM from ipw2100_net_init() which is
called by register_netdev.

CC: Dan Carpenter <error27@gmail.com>
Signed-off-by: Christoph Fritz <chf.fritz@googlemail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>