dai: intel: dmic: fix shadow variable

Fix a shadow variable build error:

dmic_nhlt.c: In function 'dai_dmic_write_coeff':
dmic_nhlt.c:57:78: error: declaration of 'base' shadows a global
declaration [-Werror=shadow]

Signed-off-by: Fabio Baltieri <fabiobaltieri@google.com>