mfd: iio: ti_am335x_adc: rename device from tiadc to TI-am335x-adc

TI-adc reads a little better compared to tiadc. And if we add am335x to
it then we have the same naming scheme as the tsc side.

Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>