r8169: add check for invalid parameter combination in rtl_set_coalesce

Realtek provided information about a HW constraint that time limit must
not be set to 0 if the frame limit is >0. Add a check for this and
reject invalid parameter combinations.

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