staging: hv: convert channel_mgmt.c to not call osd_schedule_callback

The additional abstraction is unneeded.

The three calls are assumed to not be pending simultaneously:
 - vmbus_onoffer queues work exactly once when a new channel is
   created, the channel is not attached to lists until the work
   is executed
 - vmbus_onoffer_rescind is received only when the channel is
   active it is enough to process the work once
 - free_channel is called exactly once when the channel is getting
   destroyed; I assumed that vmbus_process_rescind_offer cannot be
   pending while free_channel is called

Reviewed-By: Hank Janssen <hjanssen@microsoft.com>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: Timo Teräs <timo.teras@iki.fi>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>