ACPI: make acpi_bus_register_driver() return success/failure, not device count

acpi_bus_register_driver() should not return the number of devices claimed.
We're not asking to find devices, we're making a driver available to devices,
including hot-pluggable devices that may appear in the future.

I audited all callers of acpi_bus_register_driver(), and except asus_acpi.c
and sonypi.c (fixed in previous patches), all either ignore the return value
or test only for failure (<0).

Signed-off-by: Bjorn Helgaas <bjorn.helgaas@hp.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Len Brown <len.brown@intel.com>