[ALSA] dynamic minors (5/6): reduce maximum number of MIDI devices per card

Modules: ALSA sequencer,Generic drivers

To allow increasing the maximum number of sound cards, we have to limit
the number of sequencer clients per card because client numbers are
still allocated statically.

Reducing the number of clients to four limits the number of sequencer
MIDI ports to 1024 per card.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>