ar71xx: ag71xx: compute the RX buffer size from the maximum frame size

Currently, the AG71XX_RX_PKT_SIZE value limits the received
frame size to 1514/1516 bytes with/without a VLAN header
respectively. However the hardware limit is controlled by
the value the AG71XX_REG_MAC_MFL register which contains
the value of the max_frame_len field.

Compute the RX buffer size from the max_frame_len field
to get rid of the 1514/1516 byte limitation. Also remove
the unused AG71XX_RX_PKT_SIZE definition.

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

SVN-Revision: 39121