backlight: lp855x: Fix lp855x.h kernel-doc warnings

backlight: lp855x: Fix lp855x.h kernel-doc warnings

Add a missing struct short description and a missing leading " *" to
lp855x.h to avoid kernel-doc warnings:

Warning: include/linux/platform_data/lp855x.h:126 missing initial short
 description on line:
 * struct lp855x_platform_data
Warning: include/linux/platform_data/lp855x.h:131 bad line:
   Only valid when mode is PWM_BASED.

Fixes: 7be865ab8634 ("backlight: new backlight driver for LP855x devices")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Daniel Thompson (RISCstar) <danielt@kernel.org>
Link: https://patch.msgid.link/20251111060916.1995920-1-rdunlap@infradead.org
Signed-off-by: Lee Jones <lee@kernel.org>