lantiq: fix compilation warning in cfi_mtdset_0001

lantiq: fix compilation warning in cfi_mtdset_0001

Some write buffer functions are not used when FORCE_WORD_WRITE is set to 1.
So the compile warning messages are output if FORCE_WORD_WRITE is 1. To
resolve this disable the write buffer functions if FORCE_WORD_WRITE is 1.

This fix compilation warning:
drivers/mtd/chips/cfi_cmdset_0001.c:307:13: warning: 'fixup_use_write_buffers' defined but not used [-Wunused-function]
  307 | static void fixup_use_write_buffers(struct mtd_info *mtd)
      |             ^~~~~~~~~~~~~~~~~~~~~~~
cc1: all warnings being treated as errors

Signed-off-by: Aleksander Jan Bajkowski <olek2@wp.pl>