i2c: qcom-geni: change i2c_master_hub to static

i2c_master_hub is only used in i2c-qcom-geni.c now,
change it to static.

Fixes: cacd9643 ("i2c: qcom-geni: add support for I2C Master Hub variant")
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Reviewed-by: Bjorn Andersson <andersson@kernel.org>
Signed-off-by: Wolfram Sang <wsa@kernel.org>