mfd: cs47l90: Make DAC_AEC_CONTROL_2 readable

There is a second AEC loopback on cs47l90 so the registers for it should
be readable.

Signed-off-by: Ajit Pandey <ajit.pandey@cirrus.com>
Signed-off-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>