[bwn] implement reset improvements in preparation for PHY-N support

• Ensure we set 20MHz wide channels (hard-coded) for PHY-N.
• Change the core rese tto take a flag saying "gmode" vesus uint32_t flags. This is important for BCMA support where the "gmode" bit is different.
• Refactor out the mac-phy clock reset routine (usde by PHY-N).

Tested:

• BCM4321 (PHY-N), BCM4312 (PHY-LP)

TODO:

• Checkpoint test on PHY-G hardware, just to check.