iio:adc:berlin2-adc: use channel-array size directly

Drop the otherwise unused definition of the channel-array size and use it
directly in _probe - makes it a bit more obvious.

Signed-off-by: Hartmut Knaack <knaack.h@gmx.de>
Acked-by: Antoine Tenart <antoine.tenart@free-electrons.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>