mtd: Few follow up cleanups for Smartmedia/xD support

* Test results of few functions that were declared with __must_check
* Fix bogus gcc warning about uinitialized variable 'ret'
* Remove unused variable from mtdblock_remove_dev
* Don't use deprecated DMA_32BIT_MASK

Signed-off-by: Maxim Levitsky <maximlevitsky@gmail.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>