qtest/libqos: Reduce size_to_prdtl() declaration scope

qtest/libqos: Reduce size_to_prdtl() declaration scope

Since size_to_prdtl() is only used within ahci.c,
declare it statically. This removes the last use
of "inlined function with external linkage". See
previous commit and commit 9de9fa5cf2 for rationale.

Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-Id: <20240326171009.26696-4-philmd@linaro.org>