ACPI: ibm-acpi: do not use / in driver names

ibm-acpi uses sub-device names like ibm/hotkey, which get in the way of
a sysfs conversion.  Fix it to use ibm_hotkey instead.  Thanks to Zhang
Rui for noticing this.

Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>