stlink: fix alignment build warning

stlink: fix alignment build warning

The {read,write}_mem32 interface functions was asking a 32 bits buffer
but they don't need 32 bits alignment.
This will change the interface to a 8 bits buffer to remove the
alignment mismatch warning. This was causing build errors on platforms
with strict aliasing rules.

Change-Id: I338be8df5686f07a64ddb4f17c1bb494af583999
Signed-off-by: Alexandre Pereira da Silva <aletes.xgr@gmail.com>
Reviewed-on: http://openocd.zylin.com/483
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>