iio: qcom-spmi-adc5-gen3 : Update conversion and IRQ handling logic

Fix timer value for immediate conversion to the right constant
to avoid TM conversions for immediate conversion requests.
Correct TM channel offsets checked from status and data registers
in TM interrupt handler.

Change-Id: I43a9a5cf33d31029fe306e61c4bf2e80966b9b7e
Signed-off-by: Jishnu Prakash <jprakash@codeaurora.org>