thinkpad-acpi: fix BEEP ACPI handler warnings

Some ThinkPads want two arguments for BEEP, while others want just
one, causing ACPICA to log warnings like this:

ACPI Warning (nseval-0177): Excess arguments - method [BEEP] needs 1,
found 2 [20080926]

Deal with it.

Signed-off-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: Len Brown <len.brown@intel.com>