Bluetooth: Move generic mgmt command dispatcher to hci_sock.c

The mgmt.c file should be reserved purely for HCI_CHANNEL_CONTROL. The
mgmt_control() function in it is already completely generic and has a
single user in hci_sock.c. This patch moves the function there and
renames it a bit more appropriately to hci_mgmt_cmd() (as it's a command
dispatcher).

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