Staging: hv: test return value of VmbusChannelEstablishGpadl()

The return value of VmbusChannelEstablishGpadl() was not examined in
Channel.c

Signed-off-by: Bill Pemberton <wfp5p@virginia.edu>
Cc: Hank Janssen <hjanssen@microsoft.com>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>