mfd: Fix twl4030 dependencies for audio codec

The codec for Devkit8000 (TWL4030)  was not detected except
when build with CONFIG_SND_SOC_ALL_CODECS.

twl-core.c still uses the CONFIG_TWL4030_CODEC for
twl_has_codec().

In commit 57fe7251
the CONFIG_TWL4030_CODEC was renamed
into CONFIG_MFD_TWL4030_AUDIO, thatswhy the codec
was not detected.

This patch renames the CONFIG_ TWL4030_CODEC into
CONFIG_MFD_TWL4030_AUDIO in twl-core.c.

Signed-off-by: Thomas Weber <weber@corscience.de>
Acked-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>