i2c: omap: switch over to do {} while loop

this will make sure that we execute at least once.
No functional changes otherwise.

Signed-off-by: Felipe Balbi <balbi@ti.com>
Signed-off-by: Shubhrajyoti D <shubhrajyoti@ti.com>
Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>