ACPI: SBS: Fix retval warning

drivers/acpi/sbs.c: In function acpi_battery_add:
drivers/acpi/sbs.c:811: warning: ignoring return value of device_create_file,
declared with attribute warn_unused_result

Additional cleanups:
* use struct acpi_battery in acpi_battery_remove() to clean up function
calls, just like acpi_battery_add() already does.

* put braces around unregister call, as it depends on dev being not NULL.

* remove unneeded braces

Signed-off-by: Jeff Garzik <jgarzik@redhat.com>
Signed-off-by: Alexey Starikovskiy <astarikovskiy@suse.de>
Signed-off-by: Len Brown <len.brown@intel.com>