smsc95xx: Fetch whole burst with 1 URB, avoid framing errors

smsc95xx_recv() does not reassemble bursts spread over multiple URBs.
If there is a lot of broadcast traffic, the fifo will fill up to the
burst cap limit. Lowering the burst cap to the URB size ensures no packet
spans multiple urbs.
Caveat, lower limit for working burst cap is 5/33 HS/FS packets.

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>