drivers: iio: adc: Update wait times for end of conversion check

Presently, there is a wait time of upto 1s for EOC interrupt
and a polling time of nearly 300ms after it, if interrupt is not
triggered. This delay is larger than required by HW and can lead
to issues of unexpected excessive delay in SW.
Reduce the wait time for EOC interrupt to 100ms and timeout time
when polling after it to 50ms, according to HW side recommendation.

Change-Id: Ia1629311ce16810d7eb2a87a15c094d14e0457a1
Signed-off-by: Jishnu Prakash <jprakash@codeaurora.org>