isdn: fix grammar in comment

Fix grammar / typo in comment.
Signed-off-by: NGeert Uytterhoeven <geert+renesas@glider.be>
Cc: Karsten Keil <isdn@linux-pingi.de>
Signed-off-by: NJiri Kosina <jkosina@suse.cz>

drivers/isdn/i4l/isdn_net.c

@@ -58,7 +58,7 @@
  * About SOFTNET:
  * Most of the changes were pretty obvious and basically done by HE already.
  *
- * One problem of the isdn net device code is that is uses struct net_device
+ * One problem of the isdn net device code is that it uses struct net_device
  * for masters and slaves. However, only master interface are registered to
  * the network layer, and therefore, it only makes sense to call netif_*
  * functions on them.