Bluetooth: Rename LE_SCANNING_* macros

This patch renames LE_SCANNING_ENABLED and LE_SCANNING_DISABLED
macros to LE_SCAN_ENABLE and LE_SCAN_DISABLE in order to keep
the same prefix others LE scan macros have.

It also fixes le_scan_enable_req function so it uses the LE_SCAN_
ENABLE macro instead of a magic number.

Signed-off-by: Andre Guedes <andre.guedes@openbossa.org>
Acked-by: Johan Hedberg <johan.hedberg@intel.com>
Signed-off-by: Gustavo Padovan <gustavo.padovan@collabora.co.uk>