Staging: quickstart: acpi_status is unsigned

acpi_bus_register_driver() returns an int, not acpi_status.  It returns
zero on success and negative error codes on failure, but acpi_status is
unsigned.  We can just use "ret" here.

Signed-off-by: Dan Carpenter <error27@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>