ath9k: use 'struct ath_hw *' as the first argument for 'ath9k_hw_nvram_read'

The 'ath9k_hw_nvram_read' function takes a
'struct ath_common *' as its first argument.
Almost each of its caller has a 'struct ath_hw *'
parameter in their argument list, and that is
dereferenced in order to get the 'struct ath_common'
pointer.

Change the first argument of 'ath9k_hw_nvram_read'
to be a 'struct ath_hw *', and remove the dereference
calls from the callers.

Also change the type of the first argument of the
ar9300_eeprom_read_{byte,word} functions.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>