Bluetooth: Rename bt_l2cap_fixed_chan_register()

Renames routine to be clear to be used to register LE L2CAP fixed
channel only. The routine operates on le_channels list object that also
was renamed to reflect the connection type. Updates all calls to
the procedure in the stack.

Change-Id: I01348b8186a0f537532f6a7b9dc59586827b204b
Signed-off-by: Arkadiusz Lichwa <arkadiusz.lichwa@tieto.com>