usbnet: sanity check for maxpacket

maxpacket of 0 makes no sense and oopses as we need to divide
by it. Give up.

V2: fixed typo in log and stylistic issues

Signed-off-by: Oliver Neukum <oneukum@suse.com>
Reported-by:  <syzbot+76bb1d34ffa0adc03baa@syzkaller.appspotmail.com>
Reviewed-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20211021122944.21816-1-oneukum@suse.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>