staging: nvec: add clk_prepare/clk_unprepare

Use clk_prepare/clk_unprepare as required by the generic clk framework.

Cc: gregkh@linuxfoundation.org
Cc: ac100@lists.launchpad.net
Cc: devel@driverdev.osuosl.org
Signed-off-by: Prashant Gaikwad <pgaikwad@nvidia.com>
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Marc Dietrich <marvin24@gmx.de>