x86/boot: Address clang -Wimplicit-fallthrough in vsprintf()

After enabling -Wimplicit-fallthrough for the x86 boot code, clang
warns:

  arch/x86/boot/printf.c:257:3: warning: unannotated fall-through between switch labels [-Wimplicit-fallthrough]
    257 |                 case 'u':
        |                 ^

Clang is a little more pedantic than GCC, which does not warn when
falling through to a case that is just break or return. Clang's version
is more in line with the kernel's own stance in deprecated.rst, which
states that all switch/case blocks must end in either break,
fallthrough, continue, goto, or return. Add the missing break to silence
the warning.

Fixes: dd0716c2

 ("x86/boot: Add a fallthrough annotation")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Justin Stitt <justinstitt@google.com>
Link: https://lore.kernel.org/r/20240516-x86-boot-fix-clang-implicit-fallthrough-v1-1-04dc320ca07c@kernel.org

Closes: https://lore.kernel.org/oe-kbuild-all/202405162054.ryP73vy1-lkp@intel.com/