bwn(4): BWN_GPL_PHY: make compile again

compiler complains about error: comparison of different enumeration types
('bwn_phy_band_t' and 'bwn_band_t'); so use the correct enum value.

Sponsored by: The FreeBSD Foundation
PR: 287695
Reported by: (tom.mcclean gmail.com)
MFC after: 3 days