serial: 8250_core: fix coding style issue

Fix a coding style issue in 8250_core.c:

- Remove redundant NULL initialization of a global pointer

Signed-off-by: Osama Abdelkader <osama.abdelkader@gmail.com>
Reviewed-by: Jiri Slaby <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20250822095614.18108-1-osama.abdelkader@gmail.com


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