p54: refactor p54_alloc_skb

Old firmwares had no problems processing frames which filled eighth of the memory window.
However we have to be a bit more careful with fat frames when we talk to new firmwares.
Apart from that, I confess the old logic was a bit weird and not very sophisticated.

Signed-off-by: Christian Lamparter <chunkeey@web.de>
Signed-off-by: John W. Linville <linville@tuxdriver.com>