devfreq: devfreq_spdm: Fix enable/disable calls

The enable and disable calls in the spdm governor are missing
the hypervisor call.  This patch adds the hvc call.

Change-Id: Ic8f4feeb9bc0b7066b6620553725aa636c017c03
Signed-off-by: Dan Sneddon <dsneddon@codeaurora.org>