WIRELESS: at76c50x, remove unneeded NULL check

Stanse found that urb cannot be NULL in at76_rx_tasklet because it is
dereferenced earlier, so remove the unneeded check.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Signed-off-by: John W. Linville <linville@tuxdriver.com>