net/mlx5e: IPOIB, Fix overflowing SQ WQE memset

IPoIB WQE size is larger than a single WQEBB.  Must not fetch the WQE,
and surely not memset it, until it is guaranteed that there are enough
WQEBBs available before getting to SQ/frag edge.

Fixes: 043dc78e

 ("net/mlx5e: TX, Use actual WQE size for SQ edge fill")
Signed-off-by: Tariq Toukan <tariqt@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>