sh: Fixup __raw_read_trylock().

generic__raw_read_trylock() was broken, fix up the __raw_read_trylock()
implementation for something sensible. Taken from m32r, which has the
same use cases.
Signed-off-by: NPaul Mundt <lethal@linux-sh.org>

include/asm-sh/spinlock.h

@@ -88,7 +88,14 @@ static inline void __raw_write_unlock(raw_rwlock_t *rw)
 	__raw_spin_unlock(&rw->lock);
 }
 
-#define __raw_read_trylock(lock) generic__raw_read_trylock(lock)
+static inline int __raw_read_trylock(raw_rwlock_t *lock)
+{
+	atomic_t *count = (atomic_t*)lock;
+	if (atomic_dec_return(count) >= 0)
+		return 1;
+	atomic_inc(count);
+	return 0;
+}
 
 static inline int __raw_write_trylock(raw_rwlock_t *rw)
 {