fixup: riscv: apply review suggestions

Author: 0xllx0

riscv/src/register/mvien.rs

@@ -89,6 +89,8 @@ set_clear_csr!(
     /// Supervisor External Interrupt Enable
     , set_sext, clear_sext, 1 << 9);
 
+read_composite_csr!(super::mvienh::read().bits(), read().bits());
+
 #[cfg(test)]
 mod tests {
     use super::*;