drivers/video/backlight/aat2870_bl.c: fix setting max_current

 - Current implementation tests wrong value for setting
   aat2870_bl->max_current.

 - In the current implementation, we cannot differentiate between 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>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>