brcmfmac: fix setting primary channel for 80 MHz width

First of all it changes the way we calculate primary channel offset. If
we use e.g. 80 MHz channel with primary frequency 5180 MHz (which means
center frequency is 5210 MHz) it makes sense to calculate primary offset
as -30 MHz.
Then it fixes values we compare primary_offset with. We were comparing
offset in MHz against -2 or 2 which was resulting in picking a wrong
primary channel.

Signed-off-by: Rafał Miłecki <zajec5@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>