Bluetooth: hci_ecc: Convert to new thread API

Convert from a private task to the thread API, also add stack usage
analysis and tune-down the stack size to avoid wasting memory:

On x86:
ecc stack (real size 1280):	unused 80	usage 1136 / 1216 (93 %)

On arm:
ecc stack (real size 1280):	unused 52	usage 1128 / 1180 (95 %)

Change-Id: Iba84a5a9cb5257c1456663adf4952c5c18650f97
Signed-off-by: Johan Hedberg <johan.hedberg@intel.com>