mlxsw: spectrum_buffers: Reduce pool size on Spectrum-2

Due to an issue on Spectrum-2, in front-panel ports split four ways, 2 out
of 32 port buffers cannot be used. To work around this, the next FW release
will mark them as unused, and will report correspondingly lower total
shared buffer size. mlxsw will pick up the new value through a query to
cap_total_buffer_size resource. However the initial size for shared buffer
pool 0 is hard-coded and therefore needs to be updated.

Thus reduce the pool size by 2.7 MiB (which corresponds to 2/32 of the
total size of 42 MiB), and round down to the whole number of cells.

Fixes: fe099bf6 ("mlxsw: spectrum_buffers: Add Spectrum-2 shared buffer configuration")
Signed-off-by: Petr Machata <petrm@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>