Staging: hv: remove GetChannelOffers vmbus_driver callback

It's only ever set to one function, so just call that function instead.
Actually, that wrapper function only ever called vmbus_request_offers()
so just call that function instead, no need for a do-nothing
intermediate step here.

Cc: Haiyang Zhang <haiyangz@microsoft.com>
Cc: Hank Janssen <hjanssen@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>