power: avs: fix uninitialized error return on failed cpr_read_fuse_uV() call

Currently when the call cpr_read_fuse_uV returns an error the value in the
uninitialized variable ret is returned. Fix this by instread returning the
error value in the variable uV.

Addresses-Coverity: ("Uninitialized scalar variable")
Fixes: bf6910ab ("power: avs: Add support for CPR (Core Power Reduction)")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>