net: stmmac: fix incorrect flag check in timestamp interrupt

The driver should continue get the timestamp if STMMAC_FLAG_EXT_SNAPSHOT_EN
flag is set.

Fixes: aa5513f5

 ("net: stmmac: replace the ext_snapshot_en field with a flag")
Cc: <stable@vger.kernel.org> # 6.6
Signed-off-by: Song Yoong Siang <yoong.siang.song@intel.com>
Signed-off-by: Lai Peter Jun Ann <jun.ann.lai@intel.com>
Reviewed-by: Jacob Keller <jacob.e.keller@intel.com>
Reviewed-by: Serge Semin <fancer.lancer@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>