i2c: omap: re-factor receive/transmit data loop

re-factor the common parts to a separate function,
so that code is easier to read and understand.

No functional changes.

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>