net: orinoco: add error handling for failed kmalloc().

With flag 'GFP_ATOMIC', probability of allocation failure is more.
Add error handling after kmalloc() call to avoid null dereference.

Signed-off-by: Santosh Nayak <santoshprasadnayak@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>