leds: lm3697: Fix out-of-bound access

If both LED banks aren't used in device tree, an out-of-bounds
condition in lm3697_init occurs because of the for loop assuming that
all the banks are used.  Fix it by adding a variable that contains the
number of used banks.

Signed-off-by: Gabriel David <ultracoolguy@tutanota.com>
[removed extra rename, minor tweaks]
Signed-off-by: Pavel Machek <pavel@ucw.cz>
Cc: stable@kernel.org