mlxsw: i2c: Fix chunk size setting in output mailbox buffer

The driver reads commands output from the output mailbox. If the size
of the output mailbox is not a multiple of the transaction /
block size, then the driver will not issue enough read transactions
to read the entire output, which can result in driver initialization
errors.

Fix by determining the number of transactions using DIV_ROUND_UP().

Fixes: 3029a693 ("mlxsw: i2c: Allow flexible setting of I2C transactions size")
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>