bcm63xx: use the generic nand upgrade on Sercomm devices

bcm63xx: use the generic nand upgrade on Sercomm devices

The Sercomm devices don't need the jffs2 cleanmarkers for the kernel
partition. With the stock firmware they are only added when the
bootloader partition is upgraded.

Furthermore, on the Sercomm H500-s, the specific cfe_jffs2_upgrade_tar
function causes OOB corruption. As a result of this, combined with the
nature of the mtdsplit driver, the upgrade procedure ends in a bricked
device.

Use the generic nand_do_upgrade provided by the OpenWrt common functions.

Signed-off-by: Daniel González Cabanelas <dgcbueu@gmail.com>