greybus: connection: fix locking in gb_hd_connection_find

Fix unconditional re-enabling of interrupts in gb_hd_connection_find,
which can be called with local interrupts disabled from the USB
completion handler.

Signed-off-by: Johan Hovold <johan@hovoldconsulting.com>
Reviewed-by: Alex Elder <elder@linaro.org>
Signed-off-by: Greg Kroah-Hartman <greg@kroah.com>