ath10k: fix null deref if device crashes early

If device failed to init during early probing
(which is quite rare) it triggered driver to
compute crc before ar->firmware was ready causing
an oops.

Fixes: 3e58044b ("ath10k: print crc32 checksums for firmware and board files")
Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>