bnxt_en: Store irq coalescing timer values in micro seconds.

bnxt_en: Store irq coalescing timer values in micro seconds.

Orabug: 23221795

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>
(cherry picked from commit dfb5b894f87cb78168e04283e8d15626dc3e6d5a)
Signed-off-by: Brian Maly <brian.maly@oracle.com>