b43: N-PHY: fix "Data bus error" while working in 5 GHz

b43: N-PHY: fix "Data bus error" while working in 5 GHz

When switching from one 5 GHz channel to another 5 GHz channel we need
to make sure BPHY is still in a reset. However to access BPHY register
we have to switch to 2 GHz mode for a moment. Otherwise this may result
in "Data bus error" (noticed by Hauke with BCM43224 connected to the
SoC).

Reported-by: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>