[ALSA] usb-audio: fix parsing of SysEx messages from CME keyboards

When CME keyboards send a SysEx message (e.g. master volume), the USB
packet uses a format different from the standard format.  Parsing this
packet according to the specification corrupts the SysEx message itself
and can cause the following MIDI messages to be misinterpreted, too.
This patch adds a workaround for this case.

Signed-off-by: Clemens Ladisch <clemens@ladisch.de>
Signed-off-by: Jaroslav Kysela <perex@suse.cz>