HID: fix pb_fnmode and move it to generic HID

The apple powerbook people are used to switch the pb_fnmode
setting at runtime through writing to sysfs, altering the
module parameter value. This was broken for them in 2.6.20-rc1
when generic HID layer was introduced, as the pb_fnmode flag
was made per-hiddevice, instead of global variable.

This patch moves the pb_fnmode module parameter from usbhid module
to hid module, but apart from that retains backward compatibility
with respect to changing the mode through sysfs.

Signed-off-by: Jiri Kosina <jkosina@suse.cz>