mtd: Fix trx check after partition rename ("linux" to "firmware")

On brcm47xx (Huawei E970), I noticed that sysupgrade now tries to flash trx images which don't fit into the flash, resulting in non-booting system.

I found that mtd does trx size checking only when flashing to a partition called "linux". This patch changes this to be "firmware".

This matches the following patch which changed partition name in bcm47xxpart.c driver to "firmware":

commit 86b4d5ef68ca77ff6724ebb9bddd6b93239c87fc
Author: hauke <hauke@3c298f89-4303-0410-b956-a3cf2f4a3e73>
 Date:   Sun Dec 1 16:32:32 2013 +0000

    brcm47xx: use "firmware" partition name
    ...
Signed-off-by: Rafał Miłecki <zajec5@gmail.com>

    SVN-Revision: 38973

Signed-off-by: Mathias Adam <m.adam--openwrt@adamis.de>

SVN-Revision: 42403