rust: firmware: use core::ffi::CStr method names

Prepare for `core::ffi::CStr` taking the place of `kernel::str::CStr` by
avoid methods that only exist on the latter.

Link: Rust-for-Linux#1075
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-by: Alice Ryhl <aliceryhl@google.com>
Reviewed-by: Benno Lossin <lossin@kernel.org>
Acked-by: Danilo Krummrich <dakr@kernel.org>
Signed-off-by: Tamir Duberstein <tamird@gmail.com>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Author: tamird

rust/kernel/firmware.rs

@@ -291,7 +291,7 @@ impl<const N: usize> ModInfoBuilder<N> {
         let module_name = this.module_name;
 
         if !this.module_name.is_empty() {
-            this = this.push_internal(module_name.as_bytes_with_nul());
+            this = this.push_internal(module_name.to_bytes_with_nul());
 
             if N != 0 {
                 // Re-use the space taken by the NULL terminator and swap it with the '.' separator.