ath9k: add functions to get paprd rate mask

This removes some code duplication with le32_to_cpu. This may also be
required for QCN550x support, to provide an abstraction over the
underlying EEPROM format.

Signed-off-by: Wenli Looi <wlooi@ucalgary.ca>
Acked-by: Toke Høiland-Jørgensen <toke@toke.dk>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20220320233010.123106-7-wlooi@ucalgary.ca