usbnet: dm9601: Fix incorrect command

commit 24b1042c ("usbnet: dm9601: apply introduced usb command
APIs") removes the distiction between DM_WRITE_REG and DM_WRITE_REGS
command. The distiction is reintroduced to the driver so that the
functionality of the driver remains same.

CC: Ming Lei <ming.lei@canonical.com>
Signed-off-by: Tushar Behera <tushar.behera@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>