mfd: ti_tscadc: deal with partial activation

Fix the mfd device in the case where a subdevice might not be activated.

Signed-off-by: Pantelis Antoniou <panto@antoniou-consulting.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>