tty: n_gsm: name gsm tty device minors

Add a macro which defines the possible number of virtual devices for n_gsm
to improve code readability.

Signed-off-by: Daniel Starke <daniel.starke@siemens.com>

drivers/tty/n_gsm.c

@@ -164,6 +164,9 @@ struct gsm_dlci {
 	struct net_device *net; /* network interface, if created */
 };
 
+/* Total number of supported devices */
+#define GSM_TTY_MINORS		256
+
 /* DLCI 0, 62/63 are special or reserved see gsmtty_open */
 
 #define NUM_DLCI		64
@@ -3748,7 +3751,7 @@ static int __init gsm_init(void)
 		return status;
 	}
 
-	gsm_tty_driver = tty_alloc_driver(256, TTY_DRIVER_REAL_RAW |
+	gsm_tty_driver = tty_alloc_driver(GSM_TTY_MINORS, TTY_DRIVER_REAL_RAW |
 			TTY_DRIVER_DYNAMIC_DEV | TTY_DRIVER_HARDWARE_BREAK);
 	if (IS_ERR(gsm_tty_driver)) {
 		pr_err("gsm_init: tty allocation failed.\n");