Bluetooth: Add send_mode_rsp convenience function for mgmt.c

Several management commands have similar responses but they are not
always sent asynchronously. To enable synchronous sending (from the
managment command handler function) a send_mode_rsp function is added.

Signed-off-by: Johan Hedberg <johan.hedberg@nokia.com>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>