selftests: mlxsw: Fix test failure on Spectrum-4

Remove assumptions about shared buffer cell size and instead query the
cell size from devlink. Adjust the test to send small packets that fit
inside a single cell.

Tested on Spectrum-{1,2,3,4}.

Fixes: 47354021 ("mlxsw: spectrum: Extend to support Spectrum-4 ASIC")
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://lore.kernel.org/r/f7dfbf3c4d1cb23838d9eb99bab09afaa320c4ca.1692268427.git.petrm@nvidia.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>