vtnet: fix enabling/disabling tso

Transmit segment offloading depends on transmit checksum offloading.
Enforce that constraint. This also fixes a bug, since if_hwassist bits
are from the CSUM_ space, not from the IFCAP_ space.

Approved by: re (cperciva)
PR: 290773
Reviewed by: Timo Völker
Tested by: lg@efficientip.com
Differential Revision: https://reviews.freebsd.org/D53629

(cherry picked from commit 4c50ac68166caf7e08c5a9984d63fa91490fa50d)
(cherry picked from commit 0fb0ba51d8bc1e6673e073c1c5a02922f997f6b8)