serial: 8250: Fix section mismatch with CONFIG_MODULES=n

Without CONFIG_MODULES, __init_or_module means __init, so
serial8250_pnp_exit() and serial8250_hub6_exit() end up in .init.text.
They are also called from serial8250_exit(), which is __exit, and
modpost warns:

  WARNING: modpost: vmlinux: section mismatch in reference: serial8250_exit+0x2a (section: .exit.text) -> serial8250_pnp_exit (section: .init.text)

The exit code never runs in such a kernel, so nothing breaks at runtime.
But a function that is called from both __init and __exit code can't
carry either annotation, so drop it.

Fixes: a300273c458c ("serial: 8250: hub6: Add cleanup code")
Fixes: 4cf187f9dea9 ("serial: 8250: pnp: Annotate init and exit functions for conditional discarding")
Assisted-by: LLM
Signed-off-by: Danish Khateeb <danishkhateeb03@gmail.com>
Link: https://patch.msgid.link/20260926163506.487201-1-danishkhateeb03@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>