kernel: mtd: bcm47xxpart: improve handling TRX partition size

kernel: mtd: bcm47xxpart: improve handling TRX partition size

This is important fix for flash parsing in some corner cases. In case
of TRX subpartition with rootfs being aligned to the flash block size it
was incorrectly registered twice. Detecting & registering it as a
standalone partition was resulting in an incorrect "firmware" partition
size and possibly broken sysupgrade.

It wasn't noticed before because "rootfs" alignment depends on a kernel
size. It can happen though - depending on the configuration and the
kernel size.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
(cherry picked from commit f5195e72c0fcf2949f7d6296a5db081eb58f8e32)