ASoC: s3c2443-ac97: convert semaphore to mutex

This fixes a build failure for 2.6.31-rc4-rt1 (ARCH=arm, s3c2410_defconfig):

	  CC [M]  sound/soc/s3c24xx/s3c2443-ac97.o
	sound/soc/s3c24xx/s3c2443-ac97.c:50: warning: type defaults to 'int' in declaration of 'DECLARE_MUTEX'
	sound/soc/s3c24xx/s3c2443-ac97.c:50: warning: parameter names (without types) in function declaration
	sound/soc/s3c24xx/s3c2443-ac97.c: In function 's3c2443_ac97_read':
	sound/soc/s3c24xx/s3c2443-ac97.c:59: error: 'ac97_mutex' undeclared (first use in this function)
	sound/soc/s3c24xx/s3c2443-ac97.c:59: error: (Each undeclared identifier is reported only once
	sound/soc/s3c24xx/s3c2443-ac97.c:59: error: for each function it appears in.)
	sound/soc/s3c24xx/s3c2443-ac97.c: In function 's3c2443_ac97_write':
	sound/soc/s3c24xx/s3c2443-ac97.c:93: error: 'ac97_mutex' undeclared (first use in this function)

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>