ath10k: implement fw_checksums debugfs file

When debugging firmware problems it's useful to check checksums of each
component. Add a debugfs interface to retrieve crc32 checksums:

# cat /sys/kernel/debug/ieee80211/phy0/ath10k/fw_checksums
firmware-N.bin          cc3fb466
athwlan                 c0089f21
otp                     f3efeb4f
codeswap                00000000
board-N.bin             bebc7c08
board                   bebc7c08

Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>