libfdt: Remove duplicate FdtNode::from_mut()

The method duplicates FdtNodeMut::as_node() so remove it. Even if it's
pub, the only user seems to have been in pvmfw so replace it.

Test: m pvmfw
Test: atest liblibfdt.integration_test
Change-Id: I714a95d9a2c947e91f7bb5c86564ee13af898cbb
diff --git a/pvmfw/src/fdt.rs b/pvmfw/src/fdt.rs
index e83fe5a..1e37572 100644
--- a/pvmfw/src/fdt.rs
+++ b/pvmfw/src/fdt.rs
@@ -35,7 +35,6 @@
 use libfdt::CellIterator;
 use libfdt::Fdt;
 use libfdt::FdtError;
-use libfdt::FdtNode;
 use libfdt::FdtNodeMut;
 use log::debug;
 use log::error;
@@ -499,11 +498,8 @@
     let name = cstr!("ns16550a");
     let mut next = fdt.root_mut()?.next_compatible(name);
     while let Some(current) = next? {
-        let reg = FdtNode::from_mut(&current)
-            .reg()?
-            .ok_or(FdtError::NotFound)?
-            .next()
-            .ok_or(FdtError::NotFound)?;
+        let reg =
+            current.as_node().reg()?.ok_or(FdtError::NotFound)?.next().ok_or(FdtError::NotFound)?;
         next = if !serial_info.addrs.contains(&reg.addr) {
             current.delete_and_next_compatible(name)
         } else {