CXGBE: fix big-endian behaviour

[wma]

Summary

The setbit/clearbit pair casts the bitfield pointer

to uint8_t* which effectively treats its contents as
little-endian variable. The ffs() function accepts int as
the parameter, which is big-endian. Use uint8_t here to
avoid mismatch, as we have only 4 doorbells.

[np]

Can you please verify that the output of "sysctl dev.t6nex.0.doorbells " looks ok before commiting this? It should show the names of the bits and not just the numeric value.

[wma]

Yes, sysctl shows all doorbells just fine

root@:~ # sysctl dev.t6nex.0.doorbells
dev.t6nex.0.doorbells: 9<UDB,KDB>
root@:~ #