hwrng: refine hwrng delay for retry

Per internal test, about 5us-6us is enough for hwrng prepare
the random data. 440us udelay is much more then needed, So
reduce the udelay time for better performance.

Change-Id: I714b67ca6e4f97717db537c799936ea31c96e475
Signed-off-by: Lei wang <leiwan@codeaurora.org>