Remove lr::write() function, and add comment to lr::read() function.

Fixes #570

Author: jannic

Diff for: cortex-m/src/register/lr.rs

@@ -4,6 +4,9 @@
 use core::arch::asm;
 
 /// Reads the CPU register
+///
+/// Note that this function can't be used reliably: The value returned at least depends
+/// on whether the compiler chooses to inline the function or not.
 #[cfg(cortex_m)]
 #[inline]
 pub fn read() -> u32 {
@@ -12,9 +15,4 @@ pub fn read() -> u32 {
     r
 }
 
-/// Writes `bits` to the CPU register
-#[cfg(cortex_m)]
-#[inline]
-pub unsafe fn write(bits: u32) {
-    asm!("mov lr, {}", in(reg) bits, options(nomem, nostack, preserves_flags));
-}
+// No `write` function for the LR register, as it can't be used soundly.