ath5k: allocate ath5k_hw prior to initializing hw

We can propagate better errors upon failed hw initialization,
and set up the ath_common structure for attach purposes. This
will become important once we start using the ath_common
for read/write ops.

Signed-off-by: Luis R. Rodriguez <lrodriguez@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>