Bluetooth: Host: Issue all auto-initiated commands synchronously

This solves the following issues:
- Auto-initiation of commands is aborted if one of the issued commands
  fails
- The controller return value for a PHY update or DL update is not
  made available to the application.

As a result, the host no longer prevents the application from calling
bt_conn_le_data_len_update() and bt_conn_le_phy_update() before the
auto-initiated procedure completes. Now the controller may or may not
accept the additional command issued by the application.

It also simplifies the code:
- We no longer need to keep track of if the auto-phy update
  or auto-dl update has completed. If the controller receives
  another LE Set PHY while the procedure is pending in the LL, it can
  decide if wants to accept another procedure initiation or not.
- We no longer need to auto-initiate auto-initiation of commands in
  multiple places

Signed-off-by: Rubin Gerritsen <rubin.gerritsen@nordicsemi.no>