Bluetooth: Move code for ERTM local busy state to separate functions

The local busy state is entered and exited based on buffer status in
the socket layer (or other upper layer).  This change is in
preparation for general buffer status reports from the socket layer,
which will then be used to change the local busy status.

Signed-off-by: Mat Martineau <mathewm@codeaurora.org>
Signed-off-by: Gustavo F. Padovan <padovan@profusion.mobi>