usb: iowarrior: remove intr_idx_lock

The intr_idx_lock lock is acquired only in the completion callback of
the ->int_in_urb (iowarrior_callback()). There is only one URB that is
scheduled / completed so there can't be more than one user of the lock.
The comment says that it protects ->intr_idx and the callback is the
only place in driver that writes to it.

Remove the intr_idx_lock lock because it is superfluous.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>