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: NAlex Kiernan <alex.kiernan@gmail.com>
Reviewed-by: NSimon Glass <sjg@chromium.org>
Acked-by: NJoe Hershberger <joe.hershberger@ni.com>

drivers/fastboot/fb_nand.c

@@ -89,7 +89,7 @@ static int _fb_nand_erase(struct mtd_info *mtd, struct part_info *part)
 
 static int _fb_nand_write(struct mtd_info *mtd, struct part_info *part,
 			  void *buffer, unsigned int offset,
-			  unsigned int length, size_t *written)
+			  size_t length, size_t *written)
 {
 	int flags = WITH_WR_VERIFY;