i2c-stu300: I2C STU300 stability updates

- blk clk is enabled when an irq arrives. The clk should be enabled,
  but just to make sure.
- All error bits are handled no matter state machine state
- All irq's will run complete() except for irq's that wasn't an event.
- No more looking into status registers just in case an interrupt
  has happend and the irq handle wasn't executed.
- irq_disable/enable are now separete functions.
- clk settings calculation changed to round upwards instead of
  downwards.
- Number of address send attempts before giving up is increased to 12
  from 10 since it most times take 8 tries before getting through.

Signed-off-by: Linus Walleij <linus.walleij@stericsson.com>
Signed-off-by: Ben Dooks <ben-linux@fluff.org>