at86rf230: fix page parameter constraints

Since commit e37d2ec82a222f1819e7793a27bc052999a379fb ("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: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>