wlcore: make some of the fwlog calculations more obvious

Make some of the fwlog calculations more obvious by calculating bits
that get used and documenting what they are. Validate the read pointer
while we're at it to ensure we do not overflow the data block we have
allocated and read.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/E1lolvX-0003R3-RE@rmk-PC.armlinux.org.uk