ALSA: Add kconfig to specify the max card numbers

Currently ALSA supports up to 32 card instances when the dynamic minor
is used.  While 32 cards are usually big enough for normal use cases,
there are sometimes weird requirements with more card support.

Actually, this limitation, 32, comes from the index option, where you
can pass the bit mask to assign the card.  Other than that, we can
actually give more cards up to the minor number limits (currently 256,
which can be extended more, too).

This patch adds a new Kconfig to specify the max card numbers, and
changes a few places to accept more than 32 cards.

The only incompatibility with high card numbers would be the handling
of index option.  The index option can be still used to pass the
bitmask for card assignments, but this works only up to 32 slots.
More than 32, no bitmask style option is available but only a single
slot can be specified via index option.

Signed-off-by: Takashi Iwai <tiwai@suse.de>