shared/gatt-client: Don't use exchange MTU with BR/EDR links

Accourding to the spec MTU should ne negotiated using L2CAP channel
configuration procedures:

BLUETOOTH SPECIFICATION Version 4.2 [Vol 3, Part G] page 546:

  4.3.1 Exchange MTU

  This sub-procedure shall not be used on a BR/EDR physical link since
  the MTU size is negotiated using L2CAP channel configuration
  procedures.
1 file changed