jffs2: Fix zero sector_size when not using CONFIG_JFFS2_CMDLINE

This patch fixes a bug (?) introduced after inclusion of the new
JFFS2 code.

When not using CONFIG_JFFS2_CMDLINE, the code in cmd_jffs2.c doesn't
fill in part->sector_size (keeping it as 0), but a correct value is
needed by the code in jffs2_1pass.c. This causes all JFFS2 accesses
to be in the same place of the memory, what obviously means
impossibility to use the JFFS2 partition.

This problem is fixed in this patch by including sector size
calculation in non-CONFIG_JFFS2_CMDLINE mtdparts_init variant.

Signed-off-by: Tomasz Figa <tomasz.figa_at_gmail.com>