mwifiex: fix NULL pointer dereference in set_channel()

In set_channel() callback handler, "priv" pointer is derived from
net_device. Sometimes net_device pointer coming from the stack
is NULL which causes kernel crash.
This patch fixes the problem by deriving "priv" from wiphy
when net_device pointer is NULL.

Signed-off-by: Amitkumar Karwar <akarwar@marvell.com>
Signed-off-by: Bing Zhao <bzhao@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>