serial: 8250_port: Remove useless NULL checks

After switching to HR timers for RS485 the NULL checks for the object
inside timer functions become useless.

Remove them to avoid confusion to static analyzers.

Fixes 6e0a5de2 ("serial: 8250: Use hrtimers for rs485 delays")
Cc: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>