[FIX] hr_maintenance: Display equipment count of employee

In the employee form view and in the employee profile, the stat button
displaying the equipment count is wrong.
It counts the equipment owned by the user, not equipments assigned to the
employee.

closes odoo/odoo#38889

X-original-commit: 07bd725c
Signed-off-by: lul-odoo <LucasLefevre@users.noreply.github.com>