serial: core: use octal permissions on module param

Symbolic permissions 'S_IRUSR | S_IRGRP' are not preferred.
Use octal permissions '0440'. This also makes code shorter.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20200214114339.53897-4-andriy.shevchenko@linux.intel.com


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