soc: fsl: dpio: fix dereference of pointer p before null check

Pointer p is currently being dereferenced before it is null
checked on a memory allocation failure check. Fix this by
checking if p is null before dereferencing it.

Addresses-Coverity: ("Dereference before null check")
Fixes: 3b2abda7

 ("soc: fsl: dpio: Replace QMAN array mode with ring mode enqueue")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Li Yang <leoyang.li@nxp.com>