iio: adc: Skip workaround to read USB_IN_V channel for PM7250B

Add entry in pmic_data table to ensure the software workaround for
reading USB_IN_V channel is skipped for PM7250B, as it has the issue
fixed in hardware itself.

Change-Id: I4b40e76cb5f27104610499ba9268efd84cdf4ab7
Signed-off-by: Jishnu Prakash <jprakash@codeaurora.org>