ieee802154: fix kconfig bool/tristate muckup

menuconfig IEEE802154_DRIVERS is a bool that depends on tristate IEEE802154.
If the IEEE802154 symbol is 'm', the bool becomes 'y'.
This allows tristate symbols under IEEE802154_DRIVERS to be configured as
'y' and cause build problems.
Changing the menuconfig bool to a tristate fixes this.

drivers/built-in.o: In function `fake_scan_req':
fakehard.c:(.text+0x46d625): undefined reference to `ieee802154_nl_scan_confirm'
drivers/built-in.o: In function `fake_disassoc_req':
fakehard.c:(.text+0x46d66f): undefined reference to `ieee802154_nl_disassoc_confirm'
drivers/built-in.o: In function `fake_assoc_req':
fakehard.c:(.text+0x46d6be): undefined reference to `ieee802154_nl_assoc_confirm'

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Acked-by: Sergey Lapin <slapin@ossfans.org>
Signed-off-by: David S. Miller <davem@davemloft.net>