Fixed L2CAP Buffer Length when transmit packets

Fixed the problem of the L2CAP buffer length is incorrect than blasting
multiple small data packets from java layer. The buffer length should be
what is actually read rather than what is possible to be read.

Test: Ran the new ACTS Tests for LE CoC
Bug: 70683224
Change-Id: I6cf4d465a41e4e936420f76b49ce2dbda20d41fb