drm/i915/slpc: Fix inconsistent locked return

Fix for intel_guc_slpc_set_min_freq() warn:
inconsistent returns '&slpc->lock'.

v2: Avoid with_intel_runtime_pm with the
    internal goto/return. (Ashutosh)
    Also standardize the 'ret' if this came from
    the efficient setup. And avoid the 'unlikely'.

Fixes: 95ccf312

 ("drm/i915/guc/slpc: Allow SLPC to use efficient frequency")
Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Cc: Ashutosh Dixit <ashutosh.dixit@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220830193537.52201-1-rodrigo.vivi@intel.com