Staging: ath6kl: Fix pointer casts on 64-bit architectures

Remove unnecessary cast of firmware base address to integer before
adding an offset.

Fix direct use of sk_buff::network_header which is an offset rather
than a pointer on 64-bit architectures.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>