stlink: simplify mem R/W with SWIM

stlink: simplify mem R/W with SWIM

Thanks to API separation between SWIM and the other transports, we
can easily split the memory read/write for SWIM from the rest of
the code.
While there, use the macro STLINK_DATA_SIZE as size of data chunks
that can be read/write in SWIM. This was not implemented before.

Change-Id: I7d913c92539007e4d914480bacc0126a8f0e9705
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5531
Tested-by: jenkins