STM32L: Write partial-page flash data after full-page data

The target address for the partial data needs to be bumped past the
full page data. Otherwise, the partial data overwrites the start of
the flash block.

Change-Id: I1246b2fa8acbdb8193edcf7029309f11d1c6069c
Signed-off-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Mathias K <kesmtp@freenet.de>
Reviewed-on: http://openocd.zylin.com/555
Tested-by: jenkins
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>