jtag: declare local symbols as static

jtag: declare local symbols as static

Functions and variables that are not used outside the file should
be declared as static.

Change-Id: I58c9f5557d4809db9ccc34d32182c3797f825da1
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: http://openocd.zylin.com/5896
Tested-by: jenkins