iio:adc:berlin2-adc: use short operator format

Use augmented assignment to subtract the offset for negative temperature
values.
Specify the amount of private data to be allocated through
devm_iio_device_alloc() with sizeof(*priv), as it is shorter and common
practice in IIO.

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>