brcm80211: Delete unnecessary checks before two function calls

The functions brcmu_pkt_buf_free_skb() and usb_free_urb() test whether
their argument is NULL and then return immediately. Thus the test around
the call is not needed.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>