wifi: ath12k: add ath12k_get_num_hw()

Currently, one or more struct ath12k_hw is part of device (struct ath12k_base)
but in future, ath12k_hw would be part of device group (struct
ath12k_hw_group).  Hence, num_hw under device would be moved to device group.

To facilitate above transition, add helper ath12k_get_num_hw() to get the
number of radios per device. In future, this helper will return the number of
radios in a device group.

Tested-on: QCN9274 hw2.0 PCI WLAN.WBE.1.3.1-00173-QCAHKSWPL_SILICONZ-1
Tested-on: WCN7850 hw2.0 PCI WLAN.HMT.1.0.c5-00481-QCAHMTSWPL_V1.0_V2.0_SILICONZ-3

Signed-off-by: Karthikeyan Periyasamy <quic_periyasa@quicinc.com>
Signed-off-by: Harshitha Prem <quic_hprem@quicinc.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://patch.msgid.link/20241128165026.2618331-4-kvalo@kernel.org