Change the type of 'ndesc' from 'int' to 'uint16_t' so that descriptor index
wraparound is handled correctly.

The gory details are available here:
http://lists.freebsd.org/pipermail/freebsd-virtualization/2013-March/001119.html

This fixes a regression introduced in r247871.

Pointed out by:	Bruce Evans, Chris Torek