mf/session: Fix commands not being accepted anymore

mf/session: Ensure that the command callback does not return without clearing SESSION_FLAG_PENDING_COMMAND.

Leaving SESSION_FLAG_PENDING_COMMAND set results in no more work items being queued by session_submit_command, and even if they were, callback would return immediately due to a check at the start of the function. Thus, it is never valid to leave the function without clearing the SESSION_FLAG_PENDING_COMMAND flag.

This case can be hit by SESSION_CMD_START, and leaves the session unable to process any commands.

Fix this by ensuring that SESSION_FLAG_PENDING_COMMAND is always cleared before the command callback returns.