leds: rgb: mt6370: Fix implicit declaration for FIELD_GET

0-DAY CI Kernel Test Service reported the implicit declaration error below:

drivers/leds/rgb/leds-mt6370-rgb.c: In function'mt6370_check_vendor_info':
>> drivers/leds/rgb/leds-mt6370-rgb.c:889:15: error: implicit  declaration
   of function 'FIELD_GET' [-Werror=implicit-function-declaration]
  889 |         vid = FIELD_GET(MT6370_VENDOR_ID_MASK, devinfo);
      |

Add the missing header 'bitfield.h' to fix it.
Reported-by: kernel test robot <lkp@intel.com>
Link: https://lore.kernel.org/oe-kbuild-all/202303171729.CcgyFx17-lkp@intel.com/
Fixes: 55a8a5c16eb3 ("leds: rgb: mt6370: Add MediaTek MT6370 current sink type LED Indicator support")
Signed-off-by: ChiYuan Huang <cy_huang@richtek.com>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/1679067760-19098-1-git-send-email-cy_huang@richtek.com

drivers/leds/rgb/leds-mt6370-rgb.c

@@ -7,6 +7,7 @@
  *   Alice Chen <alice_chen@richtek.com>
  */
 
+#include <linux/bitfield.h>
 #include <linux/bitops.h>
 #include <linux/kernel.h>
 #include <linux/leds.h>