ALSA: line6: Reduce superfluous spinlock in midi.c

The midi_transmit_lock is used always inside the send_urb_lock, thus
it doesn't play any role.  Let's kill it.  Also, rename
"send_urb_lock" as a more simple name "lock" since this is the only
lock for midi.

Tested-by: Chris Rorvick <chris@rorvick.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>