mfd: wm5102: Remove registers for output 3R from readable list

The earpiece on 5102 is mono, thus there is no output 3R. Remove the
registers associated with the unused right channel from the readable
registers list.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>