BUG/MINOR: mqtt: fix PUBLISH flags validation that want all bits to be set

BUG/MINOR: mqtt: fix PUBLISH flags validation that want all bits to be set

The definition of the PUBLISH message type indicates that the LSB are
independent, but uses a value of 0xF that clearly shows an attempt to
use a mask instead, but it results in all messages not having all flags
set to be rejected. A sane approach would have been to check for a mask
and an expected value. Let's just add a special case for it in function
mqtt_read_fixed_hdr() since that's for a single message type.

This can be backported anywhere.