mtd: bcm47xxpart: fix parsing first block after aligned TRX

commit bd5d2131 upstream.

After parsing TRX we should skip to the first block placed behind it.
Our code was working only with TRX with length not aligned to the
blocksize. In other cases (length aligned) it was missing the block
places right after TRX.

This fixes calculation and simplifies the comment.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Amit Pundir <amit.pundir@linaro.org>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>