mwifiex: add a cfg80211 .get_tx_power operation callback

The mwifiex driver implements a cfg80211 .set_tx_power operation handler
but doesn't have the inverse .get_tx_power callback.

This not only has the effect that the Tx power can't be reported to user
space tools such as iwconfig and iwlist but also that the wireless core
prints a warning when a new wiphy is created due an cfg80211 operation
being implemented without its counterpart.

After this patch, the Tx power is properly reported to user-space tools:

$ iwlist mlan0 txpower
mlan0     unknown transmit-power information.

          Current Tx-Power=13 dBm       (19 mW)

and also the following warning isn't shown anymore on the driver probe:

WARNING: CPU: 3 PID: 127 at net/wireless/core.c:366 wiphy_new_nm+0x66c/0x6ac
Modules linked in: mwifiex_sdio mwifiex
CPU: 3 PID: 127 Comm: kworker/3:1 Tainted: G        W       4.7.0-rc1-next-20160531-00006-g569df5b983f3
Hardware name: SAMSUNG EXYNOS (Flattened Device Tree)
Workqueue: events request_firmware_work_func
[<c010e1ac>] (unwind_backtrace) from [<c010af38>] (show_stack+0x10/0x14)
[<c010af38>] (show_stack) from [<c0323b9c>] (dump_stack+0x88/0x9c)
[<c0323b9c>] (dump_stack) from [<c011a828>] (__warn+0xe8/0x100)
[<c011a828>] (__warn) from [<c011a8f0>] (warn_slowpath_null+0x20/0x28)
[<c011a8f0>] (warn_slowpath_null) from [<c06a42d4>] (wiphy_new_nm+0x66c/0x6ac)
[<c06a42d4>] (wiphy_new_nm) from [<bf1c24cc>] (mwifiex_register_cfg80211+0x28/0x3f0 [mwifiex])
[<bf1c24cc>] (mwifiex_register_cfg80211 [mwifiex]) from [<bf1a0018>] (mwifiex_fw_dpc+0x2b0/0x474 [mwifiex])
[<bf1a0018>] (mwifiex_fw_dpc [mwifiex]) from [<c040eb74>] (request_firmware_work_func+0x30/0x58)
[<c040eb74>] (request_firmware_work_func) from [<c012fe90>] (process_one_work+0x124/0x338)
[<c012fe90>] (process_one_work) from [<c01300dc>] (worker_thread+0x38/0x4d4)
[<c01300dc>] (worker_thread) from [<c01353b8>] (kthread+0xdc/0xf4)
[<c01353b8>] (kthread) from [<c0107978>] (ret_from_fork+0x14/0x3c)

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Tested-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>