ath10k_pci_wait() didn't notify any errors to callers, it
just printed a warning so add proper error handling. This fixes
a crash Ben reported:

ath10k: MSI-X interrupt handling (8 intrs)
ath10k: Unable to wakeup target
ath10k: target took longer 5000 us to wake up (awake count 1)
ath10k: Failed to get pcie state addr: -16
ath10k: early firmware event indicated
BUG: unable to handle kernel NULL pointer dereference at 0000000000000004
IP: [<ffffffffa06ae46c>] ath10k_ce_completed_send_next+0x47/0x122 [ath10k_pci]
Reported-by: NBen Greear <greearb@candelatech.com>
Signed-off-by: NKalle Valo <kvalo@qca.qualcomm.com>