drivers: hv: remove redundant assignment to pointer primary_channel

The pointer primary_channel is being assigned with a value that is never
used. The assignment is redundant and can be removed.  Move the
definition of primary_channel to a narrower scope.

Addresses-Coverity: ("Unused value")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Link: https://lore.kernel.org/r/20200414152343.243166-1-colin.king@canonical.com
[ wei: move primary_channel and update commit message ]
Signed-off-by: Wei Liu <wei.liu@kernel.org>