Bluetooth: trivial: Remove sparse warnings

Fix sparse warnings related to incorrect type in assignment and static
symbol.  Also use const keyword. Warnings are shown below:

...
net/bluetooth/mgmt.c:305:28: warning: incorrect type in assignment
(different base types)
net/bluetooth/mgmt.c:305:28:    expected unsigned short [usertype] *opcode
net/bluetooth/mgmt.c:305:28:    got restricted __le16 *<noident>
...
net/bluetooth/mgmt.c:2609:3: warning: symbol 'mgmt_handlers' was not declared.
Should it be static?
...

Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>