msm: ipa: add support for configuring polling interval/sleep time

Using 1 millisecond Rx polling sleep makes the cpu wake up too often,
results in more power consumption.
Using 3 milliseconds is helping on MSM8940 in terms of both
power and performance. Give provision to change the polling time via
device tree entry and use 3 milliseconds for MSM8940.
And also give provision to modify the rx polling timeout via
debugfs entry for testing purposes.

Change-Id: I88de436b29c3000dcebeec4da607dff42d0609fe
Acked-by: Mohammad Javid <mjavid@qti.qualcomm.com>
Signed-off-by: Utkarsh Saxena <usaxena@codeaurora.org>