MIPS: Do not use EXTRA_CFLAGS

MIPS: Do not use EXTRA_CFLAGS

Usage of these flags has been deprecated for nearly 4 years by:

    commit f77bf01425b11947eeb3b5b54685212c302741b8
    Author: Sam Ravnborg <sam@neptun.(none)>
    Date:   Mon Oct 15 22:25:06 2007 +0200

        kbuild: introduce ccflags-y, asflags-y and ldflags-y

Moreover, these flags (at least EXTRA_CFLAGS) have been documented for command
line use. By default, gmake(1) do not override command line setting, so this is
likely to result in build failure or unexpected behavior.

Replace their usage by Kbuild's `{as,cc,ld}flags-y'.

To: linux-kernel@vger.kernel.org
Cc: Sam Ravnborg <sam@ravnborg.org>
Patchwork: https://patchwork.linux-mips.org/patch/2710/
Cc: linux-mips@linux-mips.org
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>