ath10k: document ofdm/5ghz rate offset with a macro

Don't use literal values for offsets. While at it
rename a function so it is more clear what it
checks for.

This finally takes care of the last magic
5GHz/OFDM offset.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>