mwifiex: Fixing NULL pointer dereference

Following OOPS was seen when booting with card inserted

 BUG: unable to handle kernel NULL pointer dereference at 0000004c
 IP: [<f8b7718c>] cfg80211_get_drvinfo+0x21/0x115 [cfg80211]
 *pde = 00000000
 Oops: 0000 [#1] SMP
 Modules linked in: iwl3945 iwl_legacy mwifiex_sdio mac80211 11 sdhci_pci sdhci pl2303

'ethtool' on the mwifiex device returned this OOPS as
wiphy_dev() returned NULL.

Adding missing set_wiphy_dev() call to fix the problem.

Signed-off-by: Yogesh Ashok Powar <yogeshp@marvell.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>