fsi: master: Constify hub_master_ids

The only usage of hub_master_ids is to assign its address to the
id_table field in the fsi_driver struct, which is a const pointer, so
make it const to allow the compiler to put it in read-only memory.

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Signed-off-by: Joel Stanley <joel@jms.id.au>