ath9k_hw: fix the PA predistortion rate mask

The EEPROM PAPRD rate mask fields only contain mask values for actual
rates in the low 25 bits. The upper bits are reserved for tx power
scale values. Add the proper mask definitions and use them before
writing the values to the register.

Signed-off-by: Felix Fietkau <nbd@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>