jtag: getting the JTAG speed can fail

If the JTAG speed has not been set, then it has no defined
value, add code to propagate the error.

No change to actual behavior as no new failure paths have
been introduced. This is a no-op patch to make subsequent patches
smaller.

Signed-off-by: Øyvind Harboe <oyvind.harboe@zylin.com>