ath9k: spin_lock_bh is not required within tasklet context.

Disabling BH is not required while running from a tasklet context
and so replace spin_lock_bh with just spin_lock.

Signed-off-by: Senthil Balasubramanian <senthilkumar@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>