wifi: ath12k: Remove ath12k_base::bd_api

Currently struct ath12k_base defines the member bd_api. However, this
member is only accessed within ath12k_core_fetch_bdf(). Since the
scope is local just to that one function, remove it from ath12k_base
and instead just use a local stack variable.

No functional changes, compile tested only.

Signed-off-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20231013-ath11k_bd_api-v1-2-3fefe4629706@quicinc.com