ath6kl: Fix panic when setting a channel

cfg80211 could pass a NULL net_device to the driver via the
set_channel() callback, when it receives a request to set the
device's channel. Not handling this case properly results in this panic:

BUG: unable to handle kernel NULL pointer dereference at 0000000000000cb0
IP: [<ffffffffa0374e49>] ath6kl_cfg80211_ready+0x9/0x70 [ath6kl_sdio]

Call Trace:
[<ffffffffa0374ed7>] ath6kl_set_channel+0x27/0x90 [ath6kl_sdio]
[<ffffffffa04dfd5f>] cfg80211_set_freq+0xff/0x1d0 [cfg80211]
[<ffffffffa04d3275>] ? nl80211_set_wiphy+0x85/0x660 [cfg80211]
[<ffffffffa04d3198>] __nl80211_set_channel.isra.39+0x118/0x140 [cfg80211]
[<ffffffffa04d34f3>] nl80211_set_wiphy+0x303/0x660 [cfg80211]
[<ffffffff813678d7>] ? rtnl_lock+0x17/0x20
[<ffffffffa04ca165>] ? nl80211_pre_doit+0xb5/0x150 [cfg80211]
[<ffffffff81382a45>] genl_rcv_msg+0x1d5/0x250
[<ffffffff81382870>] ? genl_rcv+0x40/0x40
[<ffffffff81381be9>] netlink_rcv_skb+0xa9/0xd0
[<ffffffff81382855>] genl_rcv+0x25/0x40
[<ffffffff811354f0>] ? might_fault+0x40/0x90
[<ffffffff81381519>] netlink_unicast+0x2d9/0x320
[<ffffffff813818e6>] netlink_sendmsg+0x2c6/0x320
[<ffffffff81343800>] ? sock_update_classid+0xb0/0x110
[<ffffffff8133f52e>] sock_sendmsg+0x10e/0x130
[<ffffffff81169933>] ? mem_cgroup_update_page_stat+0x193/0x250
[<ffffffff811354f0>] ? might_fault+0x40/0x90
[<ffffffff811354f0>] ? might_fault+0x40/0x90
[<ffffffff81135539>] ? might_fault+0x89/0x90
[<ffffffff811354f0>] ? might_fault+0x40/0x90
[<ffffffff8134d126>] ? verify_iovec+0x56/0xd0
[<ffffffff8133f9e6>] __sys_sendmsg+0x396/0x3b0
[<ffffffff8108e823>] ? up_read+0x23/0x40
[<ffffffff81040e68>] ? do_page_fault+0x208/0x4e0
[<ffffffff81193151>] ? vfsmount_lock_local_unlock+0x21/0x60
[<ffffffff811947d0>] ? mntput_no_expire+0x30/0xe0
[<ffffffff8119489f>] ? mntput+0x1f/0x30
[<ffffffff81342149>] sys_sendmsg+0x49/0x90
[<ffffffff81425482>] system_call_fastpath+0x16/0x1b

Signed-off-by: Sujith Manoharan <c_manoha@qca.qualcomm.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>