firmware: use noinline_for_stack

The comment above fw_file_size() suggests it is noinline for stack size
reasons. Use noinline_for_stack to make this more clear.

Cc: Ming Lei <ming.lei@canonical.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Cesar Eduardo Barros <cesarb@cesarb.net>
Acked-by: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>