generic: 5.15: backport bcm63xx wdt patch fixing fallthrough warning

generic: 5.15: backport bcm63xx wdt patch fixing fallthrough warning

Backport patch present upstream fixing error for bcm63xx:
drivers/watchdog/bcm63xx_wdt.c: In function 'bcm63xx_wdt_ioctl':
drivers/watchdog/bcm63xx_wdt.c:208:17: error: this statement may fall through [-Werror=implicit-fallthrough=]
  208 |                 bcm63xx_wdt_pet();
      |                 ^~~~~~~~~~~~~~~~~
drivers/watchdog/bcm63xx_wdt.c:210:9: note: here
  210 |         case WDIOC_GETTIMEOUT:
      |         ^~~~
cc1: all warnings being treated as errors

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>