[Arista] Update ip packet checksum when set to 0xffff on 720DT-48S (#…

…13088)

Why I did it
This is to fix test_forward_ip_packet_with_0xffff_chksum_tolerant test failure on 720DT-48S. IP packets with checksum set to 0xffff will be forwarded with the same checksum on this platform, instead of updating to the correct value.

How I did it
Add bcm config sai_verify_incoming_chksum=0 so that checksum is updated instead of being left unchanged when checksum is 0xffff. Note that packets with invalid checksum are still dropped with this config.

device/arista/x86_64-arista_720dt_48s/td3x2-a720dt-48s-flex.config.bcm

@@ -302,3 +302,4 @@ stable_size=0x5500000
 tdma_timeout_usec.0=15000000
 tslam_timeout_usec.0=15000000
 sai_mdio_access_clause22=1
+sai_verify_incoming_chksum=0