wifi: ath11k: thermal: don't try to register multiple times

Every time the firmware boots we call ath11k_core_qmi_firmware_ready() which
ends up calling ath11k_thermal_register(). So we try to register thermal
devices multiple times. And when we power off the firmware during
suspend/hibernation (implemented in the next patch) we get a warning in resume:

hwmon hwmon4: PM: parent phy0 should not be sleeping

Workaround this similarly like ath11k_mac_register() does by testing
ATH11K_FLAG_REGISTERED.

Tested-on: WCN6855 hw2.0 PCI WLAN.HSP.1.1-03125-QCAHSPSWPL_V1_V2_SILICONZ_LITE-3.6510.30

Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Signed-off-by: Baochen Qiang <quic_bqiang@quicinc.com>
Link: https://msgid.link/20240221024725.10057-6-quic_bqiang@quicinc.com