Set a default LE(LL) TX packet length to maximum

Set a default LE(LL) TX packet length to maximum

[Problem] N/A
[Cause & Measure] To support LE packet length extension
[Checking Method] Make a LE connection and check TX/RX size

(AS IS)
    > LL_LENGTH_REQ (DEV -> Gear)
            RX Max : Octets: 251 , Time: 2120  ms
                    TX Max : Octets: 251 , Time: 2120  ms
                    < LL_LENGTH_RSP (Gear -> DEV)
            RX Max : Octets: 251 , Time: 2120  ms
                    TX Max : Octets: 27   , Time: 328  ms

                    (TO BE)
    > LL_LENGTH_REQ (DEV -> Gear)
            RX Max : Octets: 251 , Time: 2120  ms
                    TX Max : Octets: 251 , Time: 2120  ms
                    < LL_LENGTH_RSP (Gear -> DEV)
            RX Max : Octets: 251 , Time: 2120  ms
                    TX Max : Octets: 251 , Time: 2120  ms

Change-Id: Ic193a6f07b75596c1279803cecb63711e53b0c47