rt2800usb: initialize H2M_INT_SRC register

Ralink USB driver initialize H2M_INT_SRC to 0 after firmware load, and
never touch this register later. It is not touched at all by Ralink PCI
driver.

Acked-by: Helmut Schaa <helmut.schaa@googlemail.com>
Acked-by: Ivo van Doorn <IvDoorn@gmail.com>
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>