Bluetooth: hidp: NUL terminate a string in the compat ioctl

This change is similar to commit a1616a5a

 ("Bluetooth: hidp: fix
buffer overflow") but for the compat ioctl.  We take a string from the
user and forgot to ensure that it's NUL terminated.

I have also changed the strncpy() in to strscpy() in hidp_setup_hid().
The difference is the strncpy() doesn't necessarily NUL terminate the
destination string.  Either change would fix the problem but it's nice
to take a belt and suspenders approach and do both.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>