ALSA: usb-audio: Add sanity checks for invalid EPs

USB-audio driver may set up a URB containing the fixed EP without
validating its presence for some non-class-compliant devices.  This
may end up with an oops-like kernel warning when submitted.

For avoiding it, this patch adds the call of the new sanity-check
helper for URBs.  The checks are needed only for MIDI I/O as the other
places have already some other checks.

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