mv643xx_eth: remove write-only interrupt coalescing variables

Remove the write-only ->[rt]x_int_coal members from struct
mv643xx_eth_private.  In the process, tweak the RX/TX interrupt
mitigation code so that it is compiled by default, and set the
default coalescing delays to 0 usec.

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