drivers: net: usb: pegasus: fix control urb submission

Pegasus driver used single callback for sync and async control URBs.
Special flags were employed to distinguish between both, but due to flawed
logic it didn't always work.  As a result of this change
[get|set]_registers() are now much simpler.  Async write is also leaner
and does not use single, statically allocated memory for usb_ctrlrequest,
which is another potential race when asynchronously submitting URBs.

Signed-off-by: Petko Manolov <petkan@nucleusys.com>
Signed-off-by: David S. Miller <davem@davemloft.net>