mtd: fix trx_fixup

mtd: fix trx_fixup

With the BRCM47xx image I have built (Linksys E3000), there are additional
writes following the completion of "mtd_replace_jffs2" which invalidate the
'trx_fixup' performed by 'mtd_replace_jffs2'.  Moving the 'trx_fixup' to somewhere
after all writes have completed fixes the problem.  I also noticed that 'erasesize'
used to compute 'block_offset' in 'mtd_fixtrx' is used before it is computed by
'mtd_check_open'; moving the call to 'mtd_check_open' up a few lines fixes this.

Unlike 'mtd_fixtrx', 'trx_fixup' appears to assume that the TRX header is always at
offset 0; which may be the cause of the problem described in Ticket #8960.

Signed-off-by: Nathan Hintz <nlhintz@hotmail.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@32866 3c298f89-4303-0410-b956-a3cf2f4a3e73