smb: fix smb2 header flag parsing

smb: fix smb2 header flag parsing

The bits were being parsed in the order they're displayed in Wireshark,
rather than the order they were being seen on the wire, resulting in
direction and async being 0 more often than they should be.

Instead of bits, take the 4 bytes as an le_u32 and just use bit masks to
extract what we need into a struct, I think its easier to reason about
this way when comparing to the Microsoft documentation.