HFP: Fix reading multiple AT commands from a single data packet

The `data` received in `_read_at` may have multiple commands.

This fixes `execute_command` timing out when waiting for an `OK`
response when it is in the same data buffer, e.g. during SLC
initialization: b'\r\n+BRSF: 3904\r\n\r\nOK\r\n'

bumble/hfp.py

@@ -795,6 +795,7 @@ class HfProtocol(pyee.EventEmitter):
         # Append to the read buffer.
         self.read_buffer.extend(data)
 
+        while self.read_buffer:
             # Locate header and trailer.
             header = self.read_buffer.find(b'\r\n')
             trailer = self.read_buffer.find(b'\r\n', header + 2)