Bluetooth: ISO: Fix not handling shutdown condition

In order to properly handle shutdown syscall the code shall not assume
that the how argument is always SHUT_RDWR resulting in SHUTDOWN_MASK as
that would result in poll to immediately report EPOLLHUP instead of
properly waiting for disconnect_cfm (Disconnect Complete) which is
rather important for the likes of BAP as the CIG may need to be
reprogrammed.

Fixes: ccf74f23

 ("Bluetooth: Add BTPROTO_ISO socket type")
Signed-off-by: Luiz Augusto von Dentz <luiz.von.dentz@intel.com>