ath: Add a missing world regulatory domain 0x6C

Some customers use 0x6C world regulatory domain and this patch
adds the support.

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