rndis_wlan: Remove redundant NULL check before kfree

kfree on a NULL pointer is a no-op.

Signed-off-by: Syam Sidhardhan <s.syam@samsung.com>
Acked-by: Jussi Kivilinna <jussi.kivilinna@mbnet.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>