iio: adc: ad7625: fix type mismatch in clamp() macro

iio: adc: ad7625: fix type mismatch in clamp() macro

clamp() expects compatible operand types. The period calculation uses
nanosecond constants, while the local target variable was narrower than
the upper bound expression.

Make target unsigned long and use unsigned long bounds, including
NSEC_PER_USEC for the upper limit. This keeps the operands naturally
aligned without adding casts.

Suggested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Giorgi Tchankvetadze <giorgitchankvetadze1997@gmail.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>