tests/iio_common: Fix HEX value parsing in sanitize_clamp()
tests/iio_common: Fix HEX value parsing in sanitize_clamp()

Right now iio_reg and possibly other tools are broken.
Registers and values are typically provided in hex values.
strtoul() with base 10 returns error when hex values are parsed.
So iio_reg will always clear register 0 which in most cases will
reset the device.

Signed-off-by: Michael Hennerich <38f18d03059a1bc88082b55d7ca2d9b1ebfaffe0@analog.com>

analogdevicesinc/libiio,analogdevicesinc/libiio,analogdevicesinc/libiio,analogdevicesinc/libiio