leds: leds-lm3697: Provide some missing descriptions for struct members

Fixes the following W=1 kernel build warning(s):

 drivers/leds/leds-lm3697.c:60: warning: Function parameter or member 'enabled' not described in 'lm3697_led'
 drivers/leds/leds-lm3697.c:60: warning: Function parameter or member 'num_leds' not described in 'lm3697_led'
 drivers/leds/leds-lm3697.c:84: warning: Function parameter or member 'bank_cfg' not described in 'lm3697'
 drivers/leds/leds-lm3697.c:84: warning: Function parameter or member 'num_banks' not described in 'lm3697'

Cc: Pavel Machek <pavel@ucw.cz>
Cc: Dan Murphy <dmurphy@ti.com>
Cc: linux-leds@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Pavel Machek <pavel@ucw.cz>