simplelink: use 'errno' instead of '*__errno()' for picolibc compat

Picolibc doesn't always use an __errno function, preferring to use TLS
errno variable instead as that's more efficient. But, that means
applications shouldn't dig through the original errno abstraction to
use the underlying implementation directly.

Signed-off-by: Keith Packard <keithp@keithp.com>

simplelink/kernel/zephyr/dpl/dpl.c

@@ -110,7 +110,7 @@ int dpl_set_errno(int err)
 	 * __errno() is a Zephyr function returning a pointer to the
 	 * current thread's errno variable.
 	 */
-	*__errno() = (err < 0? -err : err);
+	errno = (err < 0? -err : err);
 	return -1;
 }
 #endif