cpu/msp430: use SR alias instead of r2

The register r2 is the status register, which has the SR alias. Using
SR is a lot more readable than r2.

cpu/msp430/include/irq_arch.h

@@ -44,8 +44,8 @@ __attribute__((always_inline)) static inline unsigned int irq_disable(void)
 {
  unsigned int state;
  __asm__ volatile(
- "mov.w r2, %[state]" "\n\t"
- "bic %[gie], r2" "\n\t"
+ "mov.w SR, %[state]" "\n\t"
+ "bic %[gie], SR" "\n\t"
  "nop" "\n\t"
  "and %[gie], %[state]" "\n\t"
  : [state] "=r"(state)
@@ -60,9 +60,9 @@ __attribute__((always_inline)) static inline unsigned int irq_enable(void)
 {
  unsigned int state;
  __asm__ volatile(
- "mov.w r2, %[state]" "\n\t"
+ "mov.w SR, %[state]" "\n\t"
  "nop" "\n\t"
- "bis %[gie], r2" "\n\t"
+ "bis %[gie], SR" "\n\t"
  "nop" "\n\t"
  "and %[gie], %[state]" "\n\t"
  : [state] "=r"(state)
@@ -76,7 +76,7 @@ __attribute__((always_inline)) static inline unsigned int irq_enable(void)
 __attribute__((always_inline)) static inline void irq_restore(unsigned int state)
 {
  __asm__ volatile(
- "bis %[state], r2" "\n\t"
+ "bis %[state], SR" "\n\t"
  "nop" "\n\t"
  : /* no outputs */
  : [state] "r"(state)
@@ -93,7 +93,7 @@ __attribute__((always_inline)) static inline bool irq_is_enabled(void)
 {
  unsigned int state;
  __asm__ volatile(
- "mov.w r2,%[state]" "\n\t"
+ "mov.w SR,%[state]" "\n\t"
  : [state] "=r"(state)
  : /* no inputs */
  : "memory"