i2c: brcmstb: Use complete() instead of complete_all()

There is only one waiter for the completion, therefore there
is no need to use complete_all(). Let's make that clear by
using complete() instead of complete_all().

The usage pattern of the completion is:

brcmstb_send_i2c_cmd()
  reinit_completion()
  ...
  /* initiate transfer by setting iic_enable */
  ...
  brcmstb_i2c_wait_for_completion()

Signed-off-by: Daniel Wagner <daniel.wagner@bmw-carit.de>
Reviewed-by: Kamal Dasu <kdasu.kdev@gmail.com>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>