mv643xx_eth: detect extended rx coal register field

Newer hardware has a 16-bit instead of a 14-bit RX coalescing
count field in the SDMA_CONFIG register.  This patch adds a run-time
check for which of the two we have, and adjusts further writes to the
rx coal count field accordingly.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Acked-by: Dale Farnsworth <dale@farnsworth.org>