backlight: lm3630a_bl: Remove invalid checks for unsigned int < 0

unsigned ints 'sources' and 'bank' cannot be less than LM3630A_SINK_0 (0)
and LM3630A_BANK_0 (0) respecitively, so change the logic to only check
for thier two possible valid values.

Fixes W=1 warnings:

 drivers/video/backlight/lm3630a_bl.c: In function ‘lm3630a_parse_led_sources’:
 drivers/video/backlight/lm3630a_bl.c:394:18: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits]
 394 | if (sources[i] < LM3630A_SINK_0 || sources[i] > LM3630A_SINK_1)
 | ^
 drivers/video/backlight/lm3630a_bl.c: In function ‘lm3630a_parse_bank’:
 drivers/video/backlight/lm3630a_bl.c:415:11: warning: comparison of unsigned expression < 0 is always false [-Wtype-limits]
 415 | if (bank < LM3630A_BANK_0 || bank > LM3630A_BANK_1)
 | ^

Cc: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Cc: Daniel Jeong <gshark.jeong@gmail.com>
Cc: LDD MLP <ldd-mlp@list.ti.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>