fs: fat: Fix warning in normalize_longname()

fs: fat: Fix warning in normalize_longname()

As observed with clang:
fs/fat/fat_write.c:1024:13: warning: comparison of constant 128
      with expression of type 'char' is always false
      [-Wtautological-constant-out-of-range-compare]
                if ((0x80 <= c) && (c <= 0xff))
                     ~~~~ ^  ~
fs/fat/fat_write.c:1024:25: warning: comparison of constant 255
      with expression of type 'char' is always true
      [-Wtautological-constant-out-of-range-compare]
                if ((0x80 <= c) && (c <= 0xff))
                                    ~ ^  ~~~~

Fixes: 25bb9dab14f4 ("fs: fat: check and normalize file name")
Signed-off-by: Tom Rini <trini@konsulko.com>
Signed-off-by: Alexander Graf <agraf@suse.de>