wlcore: tidy up use of fw_log.actual_buff_size

Tidy up the use of fw_log.actual_buff_size - rather than reading it
multiple times and applying the endian conversion, read it once into
actual_len and use that instead.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>
Link: https://lore.kernel.org/r/E1lolvS-0003Ql-NJ@rmk-PC.armlinux.org.uk