i2c-nforce2: Move status checking to a separate function

This is the first part of the patch that adds a function to reset the
nvidia MCP51/55 i2c controller, if something bad happens to it (e.g.
a slave sends a wrong byte count during a block transaction).

This patch just adds nforce2_check_status function. It was originally
written by Hans-Frieder Vogt.

The reason that I'm the one sending it is:
- I relied on it for the second part of the patch,
- It makes the driver code cleaner/better.

Signed-off-by: Oleg Ryjkov <olegr@olegr.ca>
Signed-off-by: Jean Delvare <khali@linux-fr.org>