[bwn] begin separating out the attach path from the SIBA specific bits.

• convert phy_getinfo() to take a "gmode" flag, rather than the siba TGSHIGH flags and then check for 2GHz. This should ensure that gmode is set correctly even on DUALPHY NICs.
• move the siba_powerup() call and the TGSHIGH decoding into a call to bwn_is_bus_siba(), and return an error if it's called on anything else. We don't yet do anything else, but when we do..

Tested:

• BCM4322, 11a STA