net/ps3_gelic_net: Fix RX sk_buff length

The Gelic Ethernet device needs to have the RX sk_buffs aligned to
GELIC_NET_RXBUF_ALIGN, and also the length of the RX sk_buffs must
be a multiple of GELIC_NET_RXBUF_ALIGN.

The current Gelic Ethernet driver was not allocating sk_buffs large
enough to allow for this alignment.

Also, correct the maximum and minimum MTU sizes, and add a new
preprocessor macro for the maximum frame size, GELIC_NET_MAX_FRAME.

Fixes various randomly occurring runtime network errors.

Fixes: 02c18891 ("ps3: gigabit ethernet driver for PS3, take3")
Signed-off-by: Geoff Levand <geoff@infradead.org>
Signed-off-by: David S. Miller <davem@davemloft.net>