shared/gatt-client: Fix discovery
shared/gatt-client: Fix discovery

The discovery shall not be interrupted in case Exchange MTU is not
supported, instead we shall continue using the default MTU as stated by
the spec:

BLUETOOTH SPECIFICATION Version 4.2 [Vol 3, Part G] page 546:
  "If the Error Response is sent by the server with the Error Code set
   to Request Not Supported , the Attribute Opcode is not supported and
   the default MTU shall be used."

silent-snowman/bluez,ComputeCycles/bluez,ComputeCycles/bluez,pkarasev3/bluez,silent-snowman/bluez,ComputeCycles/bluez,pkarasev3/bluez,ComputeCycles/bluez,silent-snowman/bluez,pkarasev3/bluez,pkarasev3/bluez,silent-snowman/bluez