thinkpad_acpi: signedness bugs getting current_mode

This needs to be signed for the error handling to work.  Valid modes are
small positive integers.

Fixes: b790ceeb ('thinkpad_acpi: Add adaptive_kbd_mode sysfs attr')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-By: Bastien Nocera <hadess@hadess.net>
Acked-by: Henrique de Moraes Holschuh <hmh@hmh.eng.br>
Signed-off-by: Darren Hart <dvhart@linux.intel.com>