ath9k: Do not remove header padding on RX from short frames

The 802.11 header is only padded to 32-bit boundary when the frame has
a non-zero length payload. In other words, control frames (e.g., ACK)
do not have a padding and we should not try to remove it. This fixes
monitor mode for short control frames. In addition, the hdrlen&3 use
is described in more detail to make it easier to understand how the
padding length is calculated.

Signed-off-by: Jouni Malinen <jouni.malinen@atheros.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>