iio: adc: ad7793: replace usleep_range() with fsleep()

The AD7792/AD7793 datasheet (Rev. B, page 25, RESET section)
says: "When a reset is initiated, the user must allow a period
of 500 us before accessing any of the on-chip registers."

Use fsleep(500) instead of usleep_range(500, 2000). The 500 us
minimum stays the same; fsleep() picks the upper slack itself
(about +25% on a default config -- narrower than the original
2000 us).

Add a code comment with the datasheet reference so the "why"
of the wait is visible at the call site.

Signed-off-by: Stepan Ionichev <sozdayvek@gmail.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>

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