r8169: improve interrupt coalescing parameter handling

The chip supports only frame limits 0, 4, 8, .. 60 internally.
Returning EINVAL for all val % 4 != 0 seems to be a little bit too
unfriendly to the user. Therefore round up the frame limit to the next
supported value. In addition round up the time limit, else a very low
limit could be rounded down to 0, and interpreted as "ignore value"
by the chip.

Signed-off-by: Heiner Kallweit <hkallweit1@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>