i2c-imx: fix error handling

- Return -ETIMEDOUT on bus busy error
- Fix timeout test "time_after(jiffies, orig_jiffies + HZ / 1000)" :
  By default, HZ=100 on arm. This means that this test has no chances to
  work and may result in a dead loop. Set timeout to 500ms.
- Don't try to send a new message if we failed to transmit
  previous one. This was preventing to recover from error on my system

Signed-off-by: Arnaud Patard <apatard@mandriva.com>
Signed-off-by: Ben Dooks <ben-linux@fluff.org>