zd1211rw: correct use of usb_bulk_msg on interrupt endpoints

zd1211rw is using usb_bulk_msg() with usb_sndbulkpipe() on interrupt endpoint.
However usb_bulk_msg() internally corrects this and makes interrupt URB.

It's better to change usb_bulk_msgs in zd1211rw to usb_interrupt_msg for less
confusion.

Signed-off-by: Jussi Kivilinna <jussi.kivilinna@mbnet.fi>
Signed-off-by: John W. Linville <linville@tuxdriver.com>