Fix the missing length hint in the TX BD for the XDP transmit path.  The
length hint is required on legacy chips.

Also, simplify the code by eliminating the first_buf local variable.
tx_buf contains the same value.  The opaque value only needs to be set
on the first BD.  Fix this also for correctness.

Fixes: a7559bc8 ("bnxt: support transmit and free of aggregation buffers")
Reviewed-by: Andy Gospodarek <andrew.gospodarek@broadcom.com>
Signed-off-by: Michael Chan <michael.chan@broadcom.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>