1. current implementation tests wrong value for setting aat2870_bl->max_current.

1. current implementation tests wrong value for setting aat2870_bl->max_current.
2. In current implementation, we cannot differentiate below 2 cases:
       a) if pdata->max_current is not set , or
       b) pdata->max_current is set to AAT2870_CURRENT_0_45 ( which is also 0 ).
   fix it by setting AAT2870_CURRENT_0_45 to be 1 and adjust the equation
   in aat2870_brightness() accordingly.

Signed-off-by: Axel Lin <axel.lin@gmail.com>
Cc: Richard Purdie <rpurdie@rpsys.net>
Cc: Samuel Ortiz <sameo@linux.intel.com>
Tested-by: Jin Park <jinyoungp@nvidia.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>