mlxsw: spectrum: Add bytes to cells helper

Buffers in the switch store packets in units called buffer cells. Add a
helper to convert from bytes to cells, so that the actual number of
cells required (result is round up) is returned.

Also, drop the SB (shared buffer) acronym from the BYTES_PER_CELL macro,
as this unit is also used in the ports' buffers and not only the
switch's shared buffer.

Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>