staging: wilc1000: simplify vif[i]->ndev accesses

With gcc-7, I got a new warning for this driver:

wilc1000/linux_wlan.c: In function 'wilc_netdev_cleanup':
wilc1000/linux_wlan.c:1224:15: error: 'vif[1]' may be used uninitialized in this function [-Werror=maybe-uninitialized]
wilc1000/linux_wlan.c:1224:15: error: 'vif[0]' may be used uninitialized in this function [-Werror=maybe-uninitialized]

A closer look at the function reveals that it's more complex than
it needs to be, given that based on how the device is created
we always get

	netdev_priv(vif->ndev) == vif

Based on this assumption, I found a few other places in the same file
that can be simplified. That code appears to be a relic from times
when the assumption above was not valid.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>