net: ipa: rework ipa_aggr_granularity_val()

The timer used for aggregation makes use of an internal 32 KHz clock.
The granularity of the timer is programmed by a field whose value is
computed by ipa_aggr_granularity_val().  Redefine the way that value
is computed by using a new TIMER_FREQUENCY constant representing the
underlying clock frequency.

Add two BUILD_BUG_ON() calls to ensure the value used is valid.

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