can-utils: remove strict-aliasing compiler warning

GitHub user 'crossband' raised an issue regarding the strict-aliasing compiler
warning in his specific setup: https://github.com/linux-can/can-utils/issues/42

In fact memcpy() and memset() are a better solution than the former pointer
magics, so remove the issues and the compiler warning flag too.

Reported-by: crossband (https://github.com/crossband)
Signed-off-by: Oliver Hartkopp <socketcan@hartkopp.net>

slcanpty.c

@@ -246,7 +246,7 @@ rx_restart:
 	else
 		ptr = 9; /* dlc position Tiiiiiiiid */
 
-	*(unsigned long long *) (&frame.data) = 0ULL; /* clear data[] */
+	memset(&frame.data, 0, 8); /* clear data[] */
 
 	if ((cmd | 0x20) == 'r' && buf[ptr] != '0') {