ffmemless: document rumble default magnitude

Added missing comment for rumble _MAGNITUDE. The default value
for it is taken from NGF_DEFAULT_RMAGNITUDE in case the parameter
is not defined for an effect.

Signed-off-by: Kalle Jokiniemi <kalle.jokiniemi@jollamobile.com>
ffmemless: Change hardcoded direction values to macro constants

There are macro constants available for direction variable in
the ffmemless library part. Take those to use instead of hard-
coded values.

Signed-off-by: Kalle Jokiniemi <kalle.jokiniemi@jollamobile.com>