serial: 8520_port: Fix function param documentation

serial: 8520_port: Fix function param documentation

The parameter is named p while the documentation talks about up.
Fix the doc to be in line with the code.

Fixes: 058bc104f7ca ("serial: 8250: Generalize rs485 software emulation")
Suggested-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Heiko Stuebner <heiko.stuebner@theobroma-systems.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20200517215610.2131618-2-heiko@sntech.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>