[media] msi2500: remove unneeded local pointer on msi2500_isoc_init()

There is no need to keep local copy of usb_device pointer as we
have same pointer stored and available easily from device state.

Signed-off-by: Antti Palosaari <crope@iki.fi>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>