staging: gigaset: fix general protection fault on probe

Fix a general protection fault when accessing the endpoint descriptors
which could be triggered by a malicious device due to missing sanity
checks on the number of endpoints.

Reported-by:  <syzbot+35b1c403a14f5c89eba7@syzkaller.appspotmail.com>
Fixes: 07dc1f9f ("[PATCH] isdn4linux: Siemens Gigaset drivers - M105 USB DECT adapter")
Cc: stable <stable@vger.kernel.org>     # 2.6.17
Cc: Hansjoerg Lipp <hjlipp@web.de>
Cc: Tilman Schmidt <tilman@imap.cc>
Signed-off-by: Johan Hovold <johan@kernel.org>
Link: https://lore.kernel.org/r/20191202085610.12719-2-johan@kernel.org


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>