ACPI: thinkpad-acpi: fix oops when a module parameter has no value

set_ibm_param() could OOPS with a NULL pointer derreference if one did not give
any values for a module parameter it handles.  This would, of course, cause all
sort of trouble for future modprobing and require a reboot to clean up
properly.

Fix it by returning -EINVAL if no values are given for the parameter, and also
avoid any nastyness from BUG_ON while at it.

How to reproduce: modprobe thinkpad-acpi brightness

Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Tested-by: Mike Kershaw <dragorn@kismetwireless.net>
Signed-off-by: Len Brown <len.brown@intel.com>