liquidio: make the spinlock octeon_devices_lock static

octeon_devices_lock can be made static as it does not need to be
in global scope.

Cleans up sparse warning: "warning: symbol 'octeon_devices_lock'
was not declared. Should it be static?"

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: David S. Miller <davem@davemloft.net>