wifi: ath12k: mhi: fix potential memory leak in ath12k_mhi_register()

mhi_alloc_controller() allocates a memory space for mhi_ctrl. When some
errors occur, mhi_ctrl should be freed by mhi_free_controller() and set
ab_pci->mhi_ctrl = NULL.

We can fix it by calling mhi_free_controller() when the failure happens
and set ab_pci->mhi_ctrl = NULL in all of the places where we call
mhi_free_controller().

Signed-off-by: Ma Ke <make_ruc2021@163.com>
Signed-off-by: Kalle Valo <quic_kvalo@quicinc.com>
Link: https://lore.kernel.org/r/20230922021036.3604157-1-make_ruc2021@163.com