iio:adc:ad799x: Fix reading and writing of event values, apply shift

last two bits of ADC and limit values are zero and should not be reported
(ad7993, ad7997); compare with read_raw()

event values are 10 (ad7993, ad7997) or 12 bit max., check the range on write

Signed-off-by: Peter Meerwald <pmeerw@pmeerw.net>
Cc: Stable@vger.kernel.org
Acked-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>