serial: 8250_early: do not save and restore IER in write callback

The IER has already been masked in early_serial8250_setup(), there is
no reason to save and restore it every time early_serial8250_write()
is called.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>