sched/fair: Make check_misfit_status() only compare dynamic capacities

check_misfit_status() checks for both capacity pressure & available CPUs
with higher capacity. Now that we have a sane(ish) capacity comparison
margin which is used throughout load-balance, this can be condensed into a
single check:

  capacity_greater(<root_domain max capacity>, <misfit task CPU's capacity>);

This has the added benefit of returning false if the misfit task CPU's is
heavily pressured, but there are no better candidates for migration.

Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
[ionela.voinescu@arm.com: fix conflict in check_misfit_status]