ath5k: fix misplaced default label in sifs switch

In this switch statement, the default case does not always assign
sifs.  In practice, ah->ah_bwmode cannot take values besides the
other labels, so this is not an actual problem, but it looks odd
and smatch complains thus:

    ath5k_hw_get_default_sifs() warn: missing break? reassigning 'sifs'

Silence the warning by moving default label up a line.

Signed-off-by: Bob Copeland <me@bobcopeland.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>