net: usb: mcs7830: fix non-DM ingress path

The mcs7830_recv() (non-DM) function discards good packets and tries to
process "bad" packets due to incorrect test condition.
Fix the condition and return the proper value as described in function
doc.

Signed-off-by: Uri Mashiach <uri.mashiach@compulab.co.il>
Acked-by: Igor Grinberg <grinberg@compulab.co.il>