fsl/fman: fix the pause_time test

pause_time is unsigned so it can't be less than zero.  The bug means
that we allow invalid pause-times.

Fixes: 57ba4c9b ('fsl/fman: Add FMan MAC support')
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>