ACPI: EC: remove .start() method

This patch folds the .start() method into .add().

acpi_ec_start() is always called immediately after acpi_ec_add(),
so there's no need to have it be a separate method.

Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Reviewed-by: Alex Chiang <achiang@hp.com>
CC: Alexey Starikovskiy <astarikovskiy@suse.de>
Signed-off-by: Len Brown <len.brown@intel.com>