Add button forced selection when FOSL_BTNN is present

This allows to force the the selection of the current button when activating
a SPU stream.

The behavior is inaccurate though, FOSL_BTNN is supposed to update the register
at time HLI_S_PTM. But it will be good enough for now.

Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>