ALSA: i2c/cs8427: Fix int to char conversion

Compiling with clang yields the following warning:

sound/i2c/cs8427.c:140:31: warning: implicit conversion from 'int'
to 'char' changes value from 160 to -96 [-Wconstant-conversion]
    data[0] = CS8427_REG_AUTOINC | CS8427_REG_CORU_DATABUF;
            ~ ~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~

Because CS8427_REG_AUTOINC is defined as 128, it is too big for a
char field.
So change data from char to unsigned char, that it can hold the value.

This patch does not change the generated code.

Signed-off-by: Philipp Klocke <philipp97kl@gmail.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>