iio: adc: ti-ads1298: Fix incorrect timeout comment

iio: adc: ti-ads1298: Fix incorrect timeout comment

At the lowest supported data rate of 250Hz, one conversion period is
4ms, not 40ms. The 50ms timeout is deliberately conservative to allow
for kernel scheduling latency, which can be significant under load or
on slow machines.

Fix the comment to state the correct conversion time, use "lowest sample
rate" for clarity, and explain that the extra margin exists to absorb
scheduling latency so that no one is tempted to shrink the timeout to
match the conversion period.

Also drop the redundant ret variable assignment by using the return value
of wait_for_completion_timeout() directly in the if() condition.

Signed-off-by: Md Shofiqul Islam <shofiqtest@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>