coresight-tmc-etr: Fix to correct the size of buf read in sg

The size of data to be read from the buffer is dependent on the
position of the read-write pointer. In scatter-gather mode there
is no straight forward relation between the two. This change
addresses the issue where we are incorrectly using the linear
relation and end up with an invalid buffer size.

Change-Id: I2d73af7ebb3680c0a39ae8b7ec7efb59a319dd78
Signed-off-by: Satyajit Desai <sadesai@codeaurora.org>