mv643xx_eth: avoid reading ->byte_cnt twice during receive processing

Currently, the receive processing reads ->byte_cnt twice (once to
update interface statistics and once to properly size the data area
of the received skb), but since receive descriptors live in uncached
memory, caching this value in a local variable saves one uncached
access, and increases routing performance a tiny little bit more.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>