wireless: carl9170: fix clang build warning

clang fails to eliminate some dead code with always-taken branches
when CONFIG_PROFILE_ANNOTATED_BRANCHES is set, leading to a false-positive
warning:

drivers/net/wireless/ath/carl9170/mac.c:522:3: error: variable 'power' is used uninitialized whenever 'if' condition is
      false [-Werror,-Wsometimes-uninitialized]
                BUG_ON(1);
                ^~~~~~~~~

Change both instances of BUG_ON(1) in carl9170 to the simpler BUG()
to avoid the warning.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Christian Lamparter <chunkeey@gmail.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>