cpuidle: ladder: fix ladder_do_selection() kernel-doc

make C=1 reports:

warning: Function parameter or struct member 'dev' not described in 'ladder_do_selection'

Document 'dev' for this function.

Signed-off-by: Jeff Johnson <quic_jjohnson@quicinc.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>