ar71xx: ag71xx: store descriptor packet length mask in ag71xx struct

The currently used bitmask is not correct for all SoCs.
Introduce a new field in struct ag71xx and store the
bitmask in that. Use the current value for now, it will
be adjusted for each SoCs in further patches.

Aslo use the new field directly in the ag71xx_rx_packets
and ag71xx_hard_start_xmit() functions and remove the
ag71xx_desc_pktlen() helper.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>

SVN-Revision: 39144