net: ipa: introduce ipa_qtime_val()

Create a new function ipa_qtime_val() which returns a value that
indicates what should be encoded for a register with a time field
expressed using Qtime.  Use it to factor out common code in
aggr_time_limit_encoded() and hol_block_timer_qtime_val().

Rename aggr_time_limit_encoded() and hol_block_timer_qtime_val() so
their names are both verbs ending in "encode".  Rename the "limit"
argument to the former to be "milliseconds" for consistency.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>