ACPICA: exserial.c: replace ternary operator with ACPI_MIN()

ACPICA commit 2250f71fe77396db21df805222ffcbf19e1e7896

Make the code simpler and more readable.

Link: https://github.com/acpica/acpica/commit/2250f71f


Signed-off-by: Jiangshan Yi <yijiangshan@kylinos.cn>
Signed-off-by: Bob Moore <robert.moore@intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>