rt2x00: avoid introducing a USB dependency in the rt2x00lib module

As reported by Felix:

Though protected by an ifdef, introducing an usb symbol dependency in
the rt2x00lib module is a major inconvenience for distributions that
package kernel modules split into individual packages.

Get rid of this unnecessary dependency by calling the usb related
function from a more suitable place.

Cc: Vishal Thanki <vishalthanki@gmail.com>
Reported-by: Felix Fietkau <nbd@nbd.name>
Fixes: 8b4c0009 ("rt2x00usb: Use usb anchor to manage URB")
Signed-off-by: Stanislaw Gruszka <sgruszka@redhat.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>