PM: clock_ops: fix missing clk_prepare() return value check

clk_prepare() can fail, so check its status and if it fails,
issue an error message and change the clock_entry_status to
PCE_STATUS_ERROR.

Signed-off-by: Aditya Pakki <pakki001@umn.edu>
[ rjw: Subject ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>