stlink: issue error for stm32 option writing

The stlink interface currently does not support 16bit read/writes.
Until a fix is included we issue a error that this is unsupported.

Change-Id: I4552cf2bd3b29e90ecc905325b743c08e2b92d67
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/808


Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>