ASoC: omap: McBSP: fix build breakage on OMAP1

ASoC: omap: McBSP: fix build breakage on OMAP1

After commits d13586574d373ef40acd4725c9a269daa355e412 ("OMAP: McBSP:
implement functional clock switching via clock framework") and
cf4c87abe238ec17cd0255b4e21abd949d7f811e ("OMAP: McBSP: implement
McBSP CLKR and FSR signal muxing via mach-omap2/mcbsp.c"), any OMAP1
board (such as the AMS Delta) that uses the ASoC McBSP driver will no
longer build:

sound/built-in.o: In function `omap_mcbsp_dai_set_dai_sysclk':
last.c:(.text+0x24ff8): undefined reference to `omap2_mcbsp1_mux_clkr_src'
last.c:(.text+0x2500c): undefined reference to `omap2_mcbsp1_mux_fsr_src'
make: *** [vmlinux] Error 1

Fix by defining three OMAP1-only dummy functions for
omap2_mcbsp1_mux_clkr_src(), omap2_mcbsp1_mux_fsr_src(), and
omap2_mcbsp_set_clks_src().

Normally, code that is OMAP SoC-revision-specific like this should go
under the arch/arm/*omap* directories, and get abstracted away from
drivers via struct platform_data function pointers.  This doesn't work
in this case since there doesn't appear to be any convenient way to access
struct platform_data (or something like it) in the current design of
the sound/soc/omap/omap-mcbsp.c driver.

Reported by Janusz Krzysztofik <jkrzyszt@tis.icnet.pl> and Tony Lindgren
<tony@atomide.com>.  Janusz also posted a patch to fix this at:

   http://www.spinics.net/lists/linux-omap/msg39560.html

(among other places), but the following approach seems less dependent
on compiler behavior.

This patch passes build tests for ams_delta_defconfig and omap2plus_defconfig,
but since I don't have an AMS Delta here, I can't boot test it on that
platform.

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Janusz Krzysztofik <jkrzyszt@tis.icnet.pl>
Cc: Tony Lindgren <tony@atomide.com>
Cc: Jarkko Nikula <jhnikula@gmail.com>
Cc: Peter Ujfalusi <peter.ujfalusi@nokia.com>
Cc: Mark Brown <broonie@opensource.wolfsonmicro.com>
Cc: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Tony Lindgren <tony@atomide.com>