mvtwsi: Remove unnecessary twsi_baud_rate and twsi_slave_address globals

These are used only once, so their is no need to have them global.

This also stops mvtwsi from using any bss vars making it easier to use
before dram init (to talk to the pmic to set the dram voltage).

Signed-off-by: Hans de Goede <hdegoede@redhat.com>