rt2x00: Clean up USB vendor request buffer functions.

There is no need to force the separation between a buffer USB vendor
request that does fit the CSR cache and one that doesn't onto the
callers. This is something that the rt2x00usb_vendor_request_buff
function can figure out by itself.
Combine the rt2x00usb_vendor_request_buff and
rt2x00usb_vendor_request_large_buff functions into a single one, as
both of them were equivalent for small buffers anyway.

Signed-off-by: Gertjan van Wingerde <gwingerde@gmail.com>
Signed-off-by: Ivo van Doorn <IvDoorn@gmail.com>