serial: core: Use string length for SysRq magic sequence

Compiler is not happy about using ARRAY_SIZE() in comparison to smaller type:

  CC      drivers/tty/serial/serial_core.o
.../serial_core.c: In function ‘uart_try_toggle_sysrq’:
.../serial_core.c:3222:24: warning: comparison is always false due to limited range of data type [-Wtype-limits]
 3222 |  if (++port->sysrq_seq < (ARRAY_SIZE(sysrq_toggle_seq) - 1)) {
      |                        ^

Looking at the code it appears that there is an additional weirdness,
i.e. use ARRAY_SIZE() against simple string literal. Yes, the idea probably
was to allow '\0' in the sequence, but it's impractical: kernel configuration
won't accept it to begin with followed by a comment about '\0' before
comparison in question.

Drop all these by switching to strlen() and convert code accordingly.

Note, GCC seems clever enough to calculate string length at compile time.

Fixes: 68af4317 ("serial/sysrq: Add MAGIC_SYSRQ_SERIAL_SEQUENCE")
Reviewed-by: Dmitry Safonov <0x7f454c46@gmail.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Link: https://lore.kernel.org/r/20200310174337.74109-1-andriy.shevchenko@linux.intel.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/serial/serial_core.c

@@ -3110,7 +3110,9 @@ static DECLARE_WORK(sysrq_enable_work, uart_sysrq_on);
  */
 static bool uart_try_toggle_sysrq(struct uart_port *port, unsigned int ch)
 {
-	if (ARRAY_SIZE(sysrq_toggle_seq) <= 1)
+	int sysrq_toggle_seq_len = strlen(sysrq_toggle_seq);
+
+	if (!sysrq_toggle_seq_len)
 		return false;
 
 	BUILD_BUG_ON(ARRAY_SIZE(sysrq_toggle_seq) >= U8_MAX);
@@ -3119,8 +3121,7 @@ static bool uart_try_toggle_sysrq(struct uart_port *port, unsigned int ch)
 		return false;
 	}
 
-	/* Without the last \0 */
-	if (++port->sysrq_seq < (ARRAY_SIZE(sysrq_toggle_seq) - 1)) {
+	if (++port->sysrq_seq < sysrq_toggle_seq_len) {
 		port->sysrq = jiffies + SYSRQ_TIMEOUT;
 		return true;
 	}