mwl8k: don't overwrite regulatory settings on fw reload

Currently the caps are parsed on every firmware reload, causing any
channel flags to be cleared.
When there is a firmware to interface mode mismatch, the triggered
firmware reload causes a reset of the regulatory settings, causing all
channels to become available:

root@openrouter:/# iw phy phy0 info
Wiphy phy0
        Band 1:
		(...)
                Frequencies:
                        * 2412 MHz [1] (0.0 dBm)
                        * 2417 MHz [2] (0.0 dBm)
                        * 2422 MHz [3] (0.0 dBm)
                        * 2427 MHz [4] (0.0 dBm)
                        * 2432 MHz [5] (0.0 dBm)
                        * 2437 MHz [6] (0.0 dBm)
                        * 2442 MHz [7] (0.0 dBm)
                        * 2447 MHz [8] (0.0 dBm)
                        * 2452 MHz [9] (0.0 dBm)
                        * 2457 MHz [10] (0.0 dBm)
                        * 2462 MHz [11] (0.0 dBm)
                        * 2467 MHz [12] (0.0 dBm)
                        * 2472 MHz [13] (0.0 dBm)
                        * 2484 MHz [14] (0.0 dBm)
		(...)

To prevent this, only parse the caps on the first firmware load during
hardware probe, and store them locally to know we have already parsed
them.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>