tc35815: Fix return value of tc35815_do_interrupt when NAPI enabled

Return received count correctly even if tx completed at the same time.
Currently NAPI is disabled for this driver so this patch does not fix
any real problem.

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