bootutil: Use uin32_t instead of size_t to count sectors

The change aligns type of "sector counter" to the type
used for *cnt parameter of flash_area_get_sectors.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>
diff --git a/boot/bootutil/src/bootutil_priv.h b/boot/bootutil/src/bootutil_priv.h
index ccf78e9..f7ca5c9 100644
--- a/boot/bootutil/src/bootutil_priv.h
+++ b/boot/bootutil/src/bootutil_priv.h
@@ -204,14 +204,14 @@
         struct image_header hdr;
         const struct flash_area *area;
         boot_sector_t *sectors;
-        size_t num_sectors;
+        uint32_t num_sectors;
     } imgs[BOOT_IMAGE_NUMBER][BOOT_NUM_SLOTS];
 
 #if MCUBOOT_SWAP_USING_SCRATCH
     struct {
         const struct flash_area *area;
         boot_sector_t *sectors;
-        size_t num_sectors;
+        uint32_t num_sectors;
     } scratch;
 #endif