uio_hv_generic: use correct channel in isr

Need to mask the correct sub-channel in the callback from VMBUS
isr.  Otherwise, can get in to infinite interrupt storm.

Fixes: 37b96a49 ("uio_hv_generic: support sub-channels")
Signed-off-by: Stephen Hemminger <sthemmin@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>