mfd: wm5102: Mark register write sequencer control 3 readable

During init the core checks if the wm5102 has finished starting by reading
register 0x19 and looking at the value. This read always fails since this
is not a readable register, mark it as being one. While we're at it provide
a constant for the register name (as supplied by Charles Keepax).

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