mv643xx_eth: clarify irq masking and unmasking

Replace the nondescriptive names ETH_INT_UNMASK_ALL and
ETH_INT_UNMASK_ALL_EXT by names of the actual fields being masked
and unmasked in the various writes to the interrupt mask registers.

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