ALSA: bcd2000: Add a sanity check for invalid EPs

As syzkaller spotted, currently bcd2000 driver submits a URB with the
fixed EP without checking whether it's actually available, which may
result in a kernel warning like:
  usb 1-1: BOGUS urb xfer, pipe 1 != type 3
  ------------[ cut here ]------------
  WARNING: CPU: 0 PID: 1846 at drivers/usb/core/urb.c:449
  usb_submit_urb+0xf8a/0x11d0
  Modules linked in:
  CPU: 0 PID: 1846 Comm: kworker/0:2 Not tainted
  4.14.0-rc2-42613-g1488251d1a98 #238
  Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
  Workqueue: usb_hub_wq hub_event
  Call Trace:
   bcd2000_init_device sound/usb/bcd2000/bcd2000.c:289
   bcd2000_init_midi sound/usb/bcd2000/bcd2000.c:345
   bcd2000_probe+0xe64/0x19e0 sound/usb/bcd2000/bcd2000.c:406
   usb_probe_interface+0x35d/0x8e0 drivers/usb/core/driver.c:361
   ....

This patch adds a sanity check of validity of EPs at the device
initialization phase for avoiding the call with an invalid EP.

Reported-by: Andrey Konovalov <andreyknvl@google.com>
Tested-by: Andrey Konovalov <andreyknvl@google.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>