regex/mlx5: fix minsize build

regex/mlx5: fix minsize build

Error occurs when configuring meson with --buildtype=minsize
with GCC 11.1.0:

drivers/regex/mlx5/mlx5_regex_fastpath.c:398:17: error:
‘len’ may be used uninitialized in this function [-Werror=maybe-uninitialized]
|                 complete_umr_wqe(qp, sq, &qp->jobs[mkey_job_id], sq->pi,
|                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|                                  klm_num, len);
|                                  ~~~~~~~~~~~~~
drivers/regex/mlx5/mlx5_regex_fastpath.c:315:31: note: ‘len’ was declared here
|         uint32_t klm_num = 0, len;
|                               ^~~

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>