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>

drivers/net/wireless/p54/p54.h

@@ -44,6 +44,8 @@ enum p54_control_frame_types {
 	P54_CONTROL_TYPE_BT_OPTIONS = 35
 };
 
+#define P54_MAX_CTRL_FRAME_LEN		0x1000
+
 #define P54_HDR_FLAG_CONTROL		BIT(15)
 #define P54_HDR_FLAG_CONTROL_OPSET	(BIT(15) + BIT(0))