mtd: bcm47xxpart: only register partitions if the trx header was filled

Sometimes the trx offsets are 0, in that case there is no partition and
we should not try to add one.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>
[Brian: rewrapped]
Signed-off-by: Brian Norris <computersforpeace@gmail.com>