rt2x00: Further L2 padding fixes.

Fix a couple of more bugs in the L2 padding code:
1. Compute the amount of L2 padding correctly (in 3 places).
2. Trim the skb correctly when the L2 padding has been applied.

Also introduce a central macro the compute the L2 padding size.

Signed-off-by: Gertjan van Wingerde <gwingerde@gmail.com>
Acked-by: Ivo van Doorn <ivdoorn@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
diff --git a/drivers/net/wireless/rt2x00/rt2x00.h b/drivers/net/wireless/rt2x00/rt2x00.h
index 4d841c0..194dae0 100644
--- a/drivers/net/wireless/rt2x00/rt2x00.h
+++ b/drivers/net/wireless/rt2x00/rt2x00.h
@@ -104,6 +104,12 @@
 #define GET_DURATION_RES(__size, __rate)(((__size) * 8 * 10) % (__rate))
 
 /*
+ * Determine the number of L2 padding bytes required between the header and
+ * the payload.
+ */
+#define L2PAD_SIZE(__hdrlen)	(-(__hdrlen) & 3)
+
+/*
  * Determine the alignment requirement,
  * to make sure the 802.11 payload is padded to a 4-byte boundrary
  * we must determine the address of the payload and calculate the