Correct the max SDU length used in BTIF layer

Use case: OPP Tx over L2CAP are failing due to invalid PUT packet from DUT.

Steps:
1. Make the Remote device setup with OPP server which supports MTU
   of more than 8k.
2. Send file from DUT to Remote device.
3. Transfer always fails.

Failure: OPP Tx over L2CAP file transfer fail. It will always fail
if remote supports more than 8076 as the MTU.

Root Cause: Max SDU of L2CAP layer will be communicated to the upper
layers, and same length of SDU being sent from upper layer
to BTIF layer, but because of change in the max SDU of
BTIF, it will receive less bytes than expected will lead
to form incomplete PUT request.

Fix: Correct the max SDU length used from BTIF layer to be in
sync with max SDU calculation from L2CAP layer.

Change-Id: Ie037c85e798bebf6a71c56488aae37455725028c