net: ipa: reduce aggregation time limit

Halve the time limit used when aggregation is enabled on an RX
endpoint, to half a millisecond.

Use DIV_ROUND_CLOSEST() to compute the value that represents the
time period, to get better accuracy in the event the time limit is
not an even multiple of the granularity.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>