i2c: mvtwsi: Eliminate twsi_control_flags

In a system where the initial u-boot location is genuinely NOR flash (as
opposed to RAM or a cache-line setup by a pre-bootloader) writes to the
data section are problematic. At best these writes have no effect, at
worst they put the flash memory into a status mode which changes the
executable code underneath us.

Pass around a stack variable from the top of the twsi i2c driver to
avoid writing to global data.

Signed-off-by: Chris Packham <judge.packham@gmail.com>