8139too: Allow using the largest possible MTU

This driver allows MTU up to 1518 bytes which is not enought to run
batman-adv. Simply raise the maximum packet size up to the maximum
allowed by the transmit descriptor, 1792 bytes, giving a maximum MTU
of 1774 bytes.

Signed-off-by: Alban Bedel <albeu@free.fr>
Signed-off-by: David S. Miller <davem@davemloft.net>