at86rf230: fix page parameter constraints

Since commit e37d2ec8 ("mac802154: ops:
declare channel and page as u8") the page parameter can't be below zero.
This patch fix a kbuild test robot warning. Furthermore this check
should be removed and handled by netlink 802.15.4 interface.
Signed-off-by: NAlexander Aring <alex.aring@gmail.com>
Signed-off-by: NMarcel Holtmann <marcel@holtmann.org>

drivers/net/ieee802154/at86rf230.c

@@ -1047,7 +1047,7 @@ at86rf230_channel(struct ieee802154_hw *hw, u8 page, u8 channel)
 	struct at86rf230_local *lp = hw->priv;
 	int rc;
 
-	if (page < 0 || page > 31 ||
+	if (page > 31 ||
 	    !(lp->hw->phy->channels_supported[page] & BIT(channel))) {
 		WARN_ON(1);
 		return -EINVAL;