tc35815: Use 0 RxFragSize.MinFrag value for non-packing mode

The datasheet say "When not enabling packing, the MinFrag value must
remain at 0".  Do not set value to RxFragSize register if
TC35815_USE_PACKEDBUFFER disabled.

This is not a bugfix.  No real problem reported on this.

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: David S. Miller <davem@davemloft.net>