soc: fsl: dpio: rename the enqueue descriptor variable

The struct qbman_eq_desc 'd' variable declaration is covering one of the
function parameters. This has no functional impact since this function
parameter was not used after the new declaration.
Even so, rename the variable so that we make the code more readable.

Fixes: 3b2abda7 ("soc: fsl: dpio: Replace QMAN array mode with ring mode enqueue")
Signed-off-by: Youri Querry <youri.querry_1@nxp.com>
Signed-off-by: Ioana Ciornei <ioana.ciornei@nxp.com>
Signed-off-by: Li Yang <leoyang.li@nxp.com>