mtd: fix some unallocated and NULL pointer issues. rwb->wrflush and...

mtd: fix some unallocated and NULL pointer issues.  rwb->wrflush and rwb->wrmaxblocks in rwbuffer could get unallocated values from ftl_initialize() in some configurations.  Also fixes related assert:

  up_assert: Assertion failed at file:rwbuffer.c line: 643

that can happen with the following configuration:

  CONFIG_FTL_WRITEBUFFER=y
  CONFIG_DRVR_WRITEBUFFER=y
  # CONFIG_FS_WRITABLE is not set

These problems are caused by CONFIG variable differences between the buffer layers. TODO: This is not a perfect solution. readahead support has similar issues.