i2c: mlxbf: select CONFIG_I2C_SLAVE

If this is not enabled, the interfaces used in this driver do not work:

drivers/i2c/busses/i2c-mlxbf.c:1888:3: error: implicit declaration of function 'i2c_slave_event' [-Werror,-Wimplicit-function-declaration]
                i2c_slave_event(slave, I2C_SLAVE_WRITE_REQUESTED, &value);
                ^
drivers/i2c/busses/i2c-mlxbf.c:1888:26: error: use of undeclared identifier 'I2C_SLAVE_WRITE_REQUESTED'
                i2c_slave_event(slave, I2C_SLAVE_WRITE_REQUESTED, &value);
                                       ^
drivers/i2c/busses/i2c-mlxbf.c:1890:32: error: use of undeclared identifier 'I2C_SLAVE_WRITE_RECEIVED'
                ret = i2c_slave_event(slave, I2C_SLAVE_WRITE_RECEIVED,
                                             ^
drivers/i2c/busses/i2c-mlxbf.c:1892:26: error: use of undeclared identifier 'I2C_SLAVE_STOP'
                i2c_slave_event(slave, I2C_SLAVE_STOP, &value);
                                       ^

Fixes: b5b5b320

 ("i2c: mlxbf: I2C SMBus driver for Mellanox BlueField SoC")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Khalil Blaiech <kblaiech@nvidia.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>