util/nvmutil: fix potential overflow in rw

off is signed, so converting that to unsigned
is better than converting rc (unsigned)
to signed. i had the right idea, but got
it wrong in the earlier version. this
should fix potential overflow issues.

Signed-off-by: Leah Rowe <leah@libreboot.org>

util/nvmutil/nvmutil.c

@@ -1894,7 +1894,7 @@ rw_file_exact(int fd, u8 *mem, size_t nrw,
 		rv = prw(fd,
 		    mem + (size_t)rc,
 		    nrw - (size_t)rc,
-		    off + (off_t)rc,
+		    (size_t)off + rc,
 		    rw_type, loop_eagain, loop_eintr,
 		    OFF_ERR);