Fix sign-comparison issue

Signed-off-by: Andrés Sánchez Pascual <tito97_sp@hotmail.com>
diff --git a/boot/nuttx/src/flash_map_backend/flash_map_backend.c b/boot/nuttx/src/flash_map_backend/flash_map_backend.c
index fcf6bab..ed58300 100644
--- a/boot/nuttx/src/flash_map_backend/flash_map_backend.c
+++ b/boot/nuttx/src/flash_map_backend/flash_map_backend.c
@@ -421,7 +421,7 @@
   /* Reposition the file offset from the beginning of the flash area */
 
   seekpos = lseek(dev->fd, (off_t)off, SEEK_SET);
-  if (seekpos != off)
+  if (seekpos != (off_t)off)
     {
       int errcode = errno;
 
@@ -489,7 +489,7 @@
   /* Reposition the file offset from the beginning of the flash area */
 
   seekpos = lseek(dev->fd, (off_t)off, SEEK_SET);
-  if (seekpos != off)
+  if (seekpos != (off_t)off)
     {
       int errcode = errno;