dmic: Allow use of reserved bits in CIC_CONTROL register

Since the mtl platform, the stereo bit has been removed from the
CIC_CONTROL register and now it is marked as reserved. On some platforms
(mtl and above), the configuration blob still has this bit set, causing the
dmic driver to report an error. This commit changes the behavior of the
driver to only display a warning.

Signed-off-by: Adrian Warecki <adrian.warecki@intel.com>