r8169: improve rtl_get_coalesce

Use FIELD_GET() macro to make the code better readable. In addition
change the logic to round the time limit up, not down. Reason is that
a time limit <1us would be rounded to 0 currently, what would be
interpreted as "no time limit set".

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