ath11k: mhi: remove unnecessary goto from ath11k_mhi_start()

No need to have goto for a return statement, so simplify the code. While at it,
print warning messages if power up calls fail.

No functional changes.

Tested-on: WCN6855 hw2.0 PCI WLAN.HSP.1.1-03003-QCAHSPSWPL_V1_V2_SILICONZ_LITE-2

Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20220401173042.17467-4-kvalo@kernel.org