i2c: mlxbf: incorrect base address passed during io write

Correct the base address used during io write.
This bug had no impact over the overall functionality of the read and write
transactions. MLXBF_I2C_CAUSE_OR_CLEAR=0x18 so writing to (smbus->io + 0x18)
instead of (mst_cause->ioi + 0x18) actually writes to the sc_low_timeout
register which just sets the timeout value before a read/write aborts.

Fixes: b5b5b320 (i2c: mlxbf: I2C SMBus driver for Mellanox BlueField SoC)
Reviewed-by: Khalil Blaiech <kblaiech@nvidia.com>
Signed-off-by: Asmaa Mnebhi <asmaa@nvidia.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>