ath10k: make core registering async

If ath10k was built into the kernel it could stall
booting for 120 seconds by default (60 seconds for
each firmware API variant) waiting for firmware
files before userspace was ready or filesystems
mounted.

Fix this by making the core registering
asynchronous.

This also shoves off about 1 second from boot time
on most systems since the driver is now mostly
initialized in a worker and modprobe takes very
little time to complete.

Signed-off-by: Michal Kazior <michal.kazior@tieto.com>
Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>