Bluetooth: Make bt_hci_driver instances link-time constants

Declaring these as const lets the linker generate more optimal code.
Some extra care is needed with hci_ecc.c since it was overwriting the
send callback. Now the choice of send() call is done directly in the
bt_send() function

Change-Id: Iac74f5ee9bee097bbb34c11bd13d1d886700f5cc
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>