hyperv/vmbus: Remove unnecessary callback check.

Sponsored by:   Microsoft OSTC
Differential Revision:  https://reviews.freebsd.org/D7046

hyperv/vmbus: Install different task function for batch/non-batch channels

This avoids bunch of unnecessary checks on hot path and simplifies the
channel processing.

Sponsored by:   Microsoft OSTC
Differential Revision:  https://reviews.freebsd.org/D7085