ASIX: Use only 11 bits of header for data size

The AX88772B uses only 11 bits of the header for the actual size. The other bits
are used for something else. This causes dmesg full of messages:

	asix_rx_fixup() Bad Header Length

This patch trims the check to only 11 bits. I believe on older chips, the
remaining 5 top bits are unused.

Signed-off-by: Marek Vasut <marek.vasut@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>