ath: remove camel case from struct reg_dmn_pair_mapping

Fixes a checkpatch warning in ath10k:

drivers/net/wireless/ath/ath10k/mac.c:1636: WARNING: Avoid CamelCase: <regpair->regDmnEnum>

Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>