net/mlx5e: Do not update SBCM when prio2buffer command is invalid

The shared buffer pools configuration which are stored in the SBCM
register are updated when the user changes the prio2buffer mapping.

However, in case the user desired prio2buffer change is invalid,
which can occur due to mapping a lossless priority to a not large enough
buffer, the SBCM update should not be performed, as the user command is
failed.

Thus, Perform the SBCM update only after xoff threshold calculation is
performed and the user prio2buffer mapping is validated.

Fixes: a440030d ("net/mlx5e: Update shared buffer along with device buffer changes")
Signed-off-by: Maher Sanalla <msanalla@nvidia.com>
Reviewed-by: Moshe Shemesh <moshe@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>