serial: 8250: Export ICR access helpers for internal use

Make ICR access helpers available outside 8250_port.c, however retain
them as ordinary static functions so as not to regress code generation.

This is because `serial_icr_write' is currently automatically inlined by
GCC, however `serial_icr_read' is not.  Making them both static inline
would grow code produced, e.g.:

$ i386-linux-gnu-size --format=gnu 8250_port-{old,new}.o
      text       data        bss      total filename
     15065       3378          0      18443 8250_port-old.o
     15289       3378          0      18667 8250_port-new.o

and:

$ riscv64-linux-gnu-size --format=gnu 8250_port-{old,new}.o
      text       data        bss      total filename
     16980       5306          0      22286 8250_port-old.o
     17124       5306          0      22430 8250_port-new.o

while making them external would needlessly add a new module interface
and lose the benefit from `serial_icr_write' getting inlined outside
8250_port.o.

Signed-off-by: Maciej W. Rozycki <macro@orcam.me.uk>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/alpine.DEB.2.21.2204181517500.9383@angie.orcam.me.uk


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