Bluetooth: btusb: check conditions before enabling USB ALT 3 for WBS

Some USB BT adapters don't satisfy the MTU requirement mentioned in
commit e848dbd3

 ("Bluetooth: btusb: Add support USB ALT 3 for WBS")
and have ALT 3 setting that produces no/garbled audio. Some adapters
with larger MTU were also reported to have problems with ALT 3.

Add a flag and check it and MTU before selecting ALT 3, falling back to
ALT 1. Enable the flag for Realtek, restoring the previous behavior for
non-Realtek devices.

Tested with USB adapters (mtu<72, no/garbled sound with ALT3, ALT1
works) BCM20702A1 0b05:17cb, CSR8510A10 0a12:0001, and (mtu>=72, ALT3
works) RTL8761BU 0bda:8771, Intel AX200 8087:0029 (after disabling
ALT6). Also got reports for (mtu>=72, ALT 3 reported to produce bad
audio) Intel 8087:0a2b.

Signed-off-by: Pauli Virtanen <pav@iki.fi>
Fixes: e848dbd3

 ("Bluetooth: btusb: Add support USB ALT 3 for WBS")
Tested-by: Michał Kępień <kernel@kempniu.pl>
Tested-by: Jonathan Lampérth <jon@h4n.dev>
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>