ASoC: Silence DEBUG_STRICT_USER_COPY_CHECKS=y warning

Enabling DEBUG_STRICT_USER_COPY_CHECKS causes the following
warning:

In file included from arch/x86/include/asm/uaccess.h:573,
                 from include/linux/poll.h:14,
                 from include/sound/pcm.h:29,
                 from include/sound/ac97_codec.h:31,
                 from sound/soc/soc-core.c:34:
In function 'copy_from_user',
    inlined from 'codec_reg_write_file' at
    sound/soc/soc-core.c:252:
arch/x86/include/asm/uaccess_64.h:65:
warning: call to 'copy_from_user_overflow' declared with
attribute warning: copy_from_user() buffer size is not provably
correct

presumably due to buf_size being signed causing GCC to fail to
see that buf_size can't become negative.

Acked-by: Liam Girdwood <lrg@ti.com>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>