dm9000: remove redundant ISR status clear

Since dm9000_interrupt() is already reading/clearing every set bit in
DM9000_ISR, this additional clear in dm9000_rx() (which is only called
by dm9000_interrupt()) is unnecessary and can be removed.

Signed-off-by: Andrew Ruder <andrew.ruder@elecsyscorp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>