Fix enum warning in ath_hal's ar9002

This fixes a clang 19 warning:

sys/dev/ath/ath_hal/ar9002/ar9285_btcoex.c:57:32: error: comparison of different enumeration types ('HAL_BOOL' and 'HAL_ANT_SETTING') [-Werror,-Wenum-compare]
   57 |             (AH5212(ah)->ah_diversity != HAL_ANT_VARIABLE)) {
      |              ~~~~~~~~~~~~~~~~~~~~~~~~ ^  ~~~~~~~~~~~~~~~~

The ah_diversity field of struct ath_hal_5212 is of type HAL_BOOL,
not the enum type HAL_ANT_SETTING. In other code, ah_diversity is
set to AH_TRUE whenever the related field ah_antControl is set to
HAL_ANT_VARIABLE.

It is not entirely clear to me what the intended statement is here: the
test as it is written now compares the enum value 0 to ah_diversity,
so in effect it enables the following block whenever ah_diversity is
AH_TRUE. Write it like that, to avoid the compiler warning.

MFC after: 3 days

(cherry picked from commit 36d486cc2ecdb9c290dba65bd5668b7e50d0d822)