net: gianfar_ptp: move set_fipers() to spinlock protecting area

set_fipers() calling should be protected by spinlock in
case that any interrupt breaks related registers setting
and the function we expect. This patch is to move set_fipers()
to spinlock protecting area in ptp_gianfar_adjtime().

Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>
Acked-by: Richard Cochran <richardcochran@gmail.com>
Reviewed-by: Fabio Estevam <fabio.estevam@nxp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>