fsl/fman: fix dtsec_set_tx_pause_frames

Fix a bug introduced in e06a03bd (fsl/fman: fix the pause_time test)
When pause_time is set to '0' - pause frames are disabled and
there's no need to apply dTSEC-A003 Errata workaround.

Signed-off-by: Igal Liberman <igal.liberman@freescale.com>
Signed-off-by: David S. Miller <davem@davemloft.net>