ACPI: fix sonypi ACPI driver registration to unregister on failure

Remove the assumption that acpi_bus_register_driver() returns the number of
devices claimed.  Returning the count is unreliable because devices may be
hot-plugged in the future (admittedly not applicable for this driver).

This also fixes a bug: if sonypi_acpi_driver was registered but found no
devices, sonypi_exit() did not unregister it.

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>