tty/early: make serial8250_early_{in,out} static again

Commit ed71871b ("tty/8250_early: Turn serial_in/serial_out into
weak symbols") made these routines weak to allow platform specific
Big endian override

However recent updates to core, specifically ebc5e200 ("serial:
of_serial: Support big-endian register accesses") and 6e63be3f
("serial: earlycon: Add support for big-endian MMIO accesses") means
that round about way to overide the early serial accessors is no longer
needed.

Cc: Jiri Slaby <jslaby@suse.cz>
Cc: Peter Hurley <peter@hurleysoftware.com>
Cc: Rob Herring <robh@kernel.org>
Cc: Kevin Cernekee <cernekee@gmail.com>
Acked-by: Noam Camus <noamc@ezchip.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>