fastboot: Fix parameter types in _fb_nand_write

fastboot: Fix parameter types in _fb_nand_write

Compiling on a 64 bit target the arguments to _fb_nand_write are
incompatible:

  drivers/fastboot/fb_nand.c: In function ‘_fb_nand_write’:
  drivers/fastboot/fb_nand.c:101:42: warning: passing argument 3 of ‘nand_write_skip_bad’ from incompatible pointer type [-Wincompatible-pointer-types]
    return nand_write_skip_bad(mtd, offset, &length, written,
                                          ^
  In file included from drivers/fastboot/fb_nand.c:16:0:
  include/nand.h:107:5: note: expected ‘size_t * {aka long unsigned int *}’ but argument is of type ‘unsigned int *’
   int nand_write_skip_bad(struct mtd_info *mtd, loff_t offset, size_t *length,
       ^~~~~~~~~~~~~~~~~~~

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>