net: igc: use skb_csum_is_sctp instead of protocol check

[ Upstream commit 609d29a9 ]

Using skb_csum_is_sctp is a easier way to validate it's a SCTP CRC
checksum offload packet, and yet it also makes igc support SCTP
CRC checksum offload for UDP and GRE encapped packets, just as it
does in igb driver.

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Reviewed-by: Alexander Duyck <alexanderduyck@fb.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Stable-dep-of: db0b124f ("igc: Enhance Qbv scheduling by using first flag bit")
Signed-off-by: Sasha Levin <sashal@kernel.org>