8250: uniphier: allow modular build with 8250 console

The recently added uniphier 8250 port driver supports early console
probing, and it supports being built as a module, but the combination
of the two fails to link:

ERROR: "early_serial8250_setup" [drivers/tty/serial/8250/8250_uniphier.ko] undefined!

Given that earlycon support in a loadable module makes no sense,
making that code conditional on 'MODULE' is a correct solution.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Fixes: b8d20e06 ("serial: 8250_uniphier: add earlycon support")
Acked-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>