Bluetooth: btmtk: fix ISO interface setup for single alt setting

Bluetooth: btmtk: fix ISO interface setup for single alt setting

Some MT6639 Bluetooth USB interfaces (e.g. IMC Networks 13d3:3588 on
ASUS ROG STRIX X870E-E and ProArt X870E-Creator boards) expose only a
single alternate setting (alt 0) on the ISO interface. The driver
unconditionally requests alt setting 1, which fails with EINVAL on
these devices, causing a ~20 second initialization delay and no LE
audio support.

Check the number of available alternate settings before selecting one.
If only alt 0 exists, use it; otherwise request alt 1 as before.

Closes: https://github.com/jetm/mediatek-mt7927-dkms/pull/39
Signed-off-by: Javier Tia <floss@jetm.me>
Reported-by: Ryan Gilbert <xelnaga@gmail.com>
Tested-by: Ryan Gilbert <xelnaga@gmail.com>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>