bnxt_en: Store irq coalescing timer values in micro seconds.

Don't convert these to internal hardware tick values before storing
them.  This avoids the confusion of ethtool -c returning slightly
different values than the ones set using ethtool -C when we convert
hardware tick values back to micro seconds.  Add better comments for
the hardware settings.

Also, rename the current set of coalescing fields with rx_ prefix.
The next patch will add support of tx coalescing values.

Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>