serial: sc16is7xx: Use devm_clk_get_optional()

Replace open coded variants of devm_clk_get_optional().

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20210517173415.7483-1-andriy.shevchenko@linux.intel.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>