i2c: mvtwsi: fix reading status register after interrupt

i2c: mvtwsi: fix reading status register after interrupt

The twsi_wait function reads the control register for interrupt flag,
and if interrupt flag is present, it immediately reads status register.

On our device this sometimes causes bad value being read from status
register, as if the value was not yet updated.

My theory is that the controller does approximately this:
  1. sets interrupt flag in control register,
  2. sets the value of status register,
  3. causes an interrupt

In U-Boot we do not use interrupts, so I think that it is possible that
sometimes the status register in the twsi_wait function is read between
points 1 and 2.

The bug does not appear if I add a small delay before reading status
register.

Wait 100ns (which in U-Boot currently means 1 us, because ndelay(i)
function calls udelay(DIV_ROUND_UP(i, 1000))) before reading the status
register.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Stefan Roese <sr@denx.de>
Cc: Mario Six <mario.six@gdsys.cc>
Cc: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Stefan Roese <sr@denx.de>