bnxt_en: Fix and simplify XDP transmit path

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>