hyperv/vmbus: Use default mtx for channel message queue

First of all sema_post() can't be called w/ spinlock, and the channel
message queue processing is not on hot code path, i.e. spinlock is not
necessary.

Submitted by:       Jun Su <junsu microsoft com>
Reviewed by:        sephe, Dexuan Cui <decui microsoft com>
MFC after:  1 week
Sponsored by:       Microsoft OSTC
Differential Revision:      https://reviews.freebsd.org/D5812