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.

PR: 290773
Reviewed by: Timo Völker
Tested by: lg@efficientip.com
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D53629