Limited DMA clearing to STM32F4, removed fatal error for flash loading.

Commit 0ed3907 added the clearing of DMA registers that was preventing
programming (see issue #74), however it uses hardcoded addresses of the
DMA registers on the STM32F4. This seems to prevent the flashing and
verification on STM32L1, as the registers only partly cover the range
zeroed. So the DMA clearing has been limited to the STM32F4
microcontroller.

Additionally, sometimes, typically directly after erases, a 'flash
loader run error' will occur that terminates the writing. This is not
necessary, as the writing is successfully performed by page writing
(line 1581 onwards of stlink-common.c), and so has been returned to a
error message (see issue #112). There is a comment on line 1574 (added by
Uwe Bonnes in commit 0164043f) that this may happen on blank devices,
and so the fatal error message is the incorrect response.
Merge pull request #138 from 6thimage/master

Fix for programming STM32L152