b43: fix list of 5 GHz channels for 802.11n cards

Broadcom hardware uses a bit tricky hw_value-s for 5 GHz channels,
values 184-228 are used for 4920-5140 MHz center frequencies. Normally
you expect channels 7-16 (e.g. 5060 MHz is channel 12, not 212). We never
meant to register hw_value 228 with freq 6140 MHz (5000 + 228 * 5).

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>