coresight: etb10: fixing the right amount of words to read

This patch rectifies the amount of words to read when the internal
buffer is deemed bigger than the amount of space available in the
perf ring buffer.

The amount to read is set to the amount of space in the perf ring
buffer rather than being subtracted by it.

Reported-by: Suzuki K Poulose <Suzuki.Poulose@arm.com>
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>