serial: 8250_men_mcb: Remove duplicate UAPI:serial_core inclusion

serial: 8250_men_mcb: Remove duplicate UAPI:serial_core inclusion

The UAPI serial_core.h is guaranteed to be included by in-kernel
one (with the same name). Individual drivers do not need to include
it explicitly. Remove it from the driver.

Note, it's a single driver in the entire kernel that does this.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20220830152313.14650-1-andriy.shevchenko@linux.intel.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>