CBUS: Fix reentrant issues in retu-headset driver

- Make headset bias and headset button detection enable/disable functions
  reentrant. Protect also suspend/resume callbacks since they call
  retu_headset_set_bias directly.
- Don't define flag variables as bit fields since bit operations may not be
  atomic and we access one of them from interrupt. Now other flags don't
  need any additional locking since they are not accessed from interrupt
  context.

Signed-off-by: Jarkko Nikula <jarkko.nikula@nokia.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>