Bluetooth: Add LE signaling commands handling

This patch splits the L2CAP command handling function in order to
have a clear separation between the commands related to BR/EDR and
LE. Commands and responses in the LE signaling channel are not being
handled yet, command reject is sent to all received requests. Bluetooth
Core Specification, Volume 3, Part A, section 4 defines the signaling
packets formats and allowed commands/responses over the LE signaling
channel.

Signed-off-by: Claudio Takahasi <claudio.takahasi@openbossa.org>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>