Fix mlnx_tune for CentOS 7.8 with mlnx5

[jdanders]

This commit fixes a couple things on my specific CentOS 7.8 system. The addition of 7.8 is obvious, but the other changes are things that aren't introduced by 7.8 specifically.

• The interrupt fix is because the interface name doesn't show up in /proc/interrupts on this system, and I'm guessing this is not a good universal fix.
• The adaptive rx and tx fix seems good -- I don't know if it ever worked as written.