iio: adc: ti_am335x_adc: allocating too much in probe

We should be allocating enough information for a tiadc_device struct
which is about 400 bytes but instead we allocate enough for a second
iio_dev struct which is over 2000 bytes.

Fixes: fea89e2d ("iio: adc: ti_am335x_adc: use variable names for sizeof() operator")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>