HID: cougar: Make parameter 'g6_is_space' dinamically settable

Parameter g6_is_space instructs the driver to map G6 keypresses
to KEY_SPACE (true) or to KEY_F18 (false). Make the parameter
configurable via module_param_cb to allow users to change its
value without reloading the module.

Signed-off-by: Daniel M. Lambea <dmlambea@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>