mlxsw: i2c: Utilize standard macros for dividing buffer into chunks

Use standard macro DIV_ROUND_UP() to determine the number of chunks
required for a given buffer.

Signed-off-by: Vadim Pasternak <vadimp@nvidia.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>