bnxt_en: fix firmware message length endianness

The explicit mask and shift is not the appropriate way to parse fields
out of a little endian struct. The length field is internally __le16
and the strategy employed only happens to work on little endian machines
because the offset used is actually incorrect (length is at offset 6).

Also remove the related and no longer used definitions from bnxt.h.

Fixes: 845adfe4

 ("bnxt_en: Improve valid bit checking in firmware response message.")
Signed-off-by: Edwin Peer <edwin.peer@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>