mac802154: move ieee802154_dev.c to main.c

The ieee802154_dev functionality contains various function for
allocation and registration of an ieee802154_dev. This is equal to the
net/mac80211/main.c file. This patch rename the ieee802154_dev.c to
main.c to have the same behaviour.
Signed-off-by: NAlexander Aring <alex.aring@gmail.com>
Signed-off-by: NMarcel Holtmann <marcel@holtmann.org>

net/mac802154/Makefile

 obj-$(CONFIG_MAC802154)	+= mac802154.o
-mac802154-objs		:= ieee802154_dev.o rx.o tx.o mac_cmd.o mib.o \
+mac802154-objs		:= main.o rx.o tx.o mac_cmd.o mib.o \
 			   monitor.o wpan.o llsec.o
 
 ccflags-y += -D__CHECK_ENDIAN__