ath9k: Differentiate between max combined and per chain power

The ath9k driver uses as maximum allowed txpower the constant
MAX_RATE_POWER. It is used to set a maximum txpower limit for the PHY
(which is combined txpower) and also the maximum txpower for per chain
rates. Its value 63 is derived from the maximum number the registers can
store for the per chain txpower.

The max txpower a user can set because of this is 31 dBm (floor(63 / 2)).
This also means that a device with multiple tx chains is even limited
further:

* 1 chain:  31 dBm per chain
* 2 chains: 28 dBm per chain
* 3 chains: 26 dBm per chain

This combined txpower limit of 31 dBm becomes even more problematic when
some extra antenna gain is set in the EEPROM. A high power device is then
no longer able to reach its potential limits.

Instead the code dealing with the combined txpower must use a higher limit
than 63 and only the code dealing with the per chain txpower have to use
the limit of 63. Since the antenna gain can be quite large and 8 bit
variables are often used in ath9k for txpower, a large, divisible by two
number like 254 is a good choice for this new limit.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>