Fix grabbing lock from atomic context in i2c driver

2 main changes:
- check for timeouts in the bcm2708_bsc_setup function as indicated by this comment:
      /* poll for transfer start bit (should only take 1-20 polls) */
  This implies that the setup function can now fail so account for this everywhere it's called
- Removed the clk_get_rate call from inside the setup function as it locks a mutex and that's not ok since we call it from under a spin lock.

removed dead code and update comment

fixed typo in comment