mwifiex: fix NULL pointer dereference in mwifiex_fw_dpc

We don't need to free/unregister wiphy when
mwifiex_register_cfg80211() fails. The routine internally takes
care of it. This redundant code can cause NULL pointer dereference,
for adapter->wiphy.

Reported-by: Ujjal Roy <royujjal@gmail.com>
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>