serial: 8250_early: squash wait_for_xmitr() into serial_putc()

Now, wait_for_xmitr() is only called from serial_putc(), and both
are short enough.  They can be merged into a single function.

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>