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>