platform/x86: i2c-multi-instantiate: Use the new i2c_acpi_client_count() helper

Use the new i2c_acpi_client_count() helper, this
results in a nice cleanup.

Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Link: https://lore.kernel.org/r/20210803160044.158802-4-hdegoede@redhat.com