serial: 8250: Let serial core initialise spin lock

Since the serial core handles spin lock initialisation,
let the driver rely on it.

Depends-on: f743061a85f5 ("serial: core: Initialise spin lock before use in uart_configure_port()")
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20200731123733.22754-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>