ARM: at91: at91sam9260: change at91_adc name

We can't use "at91_adc" to refer to the at91_adc driver anymore as
the name is used to match an id_table.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>