target/riscv: Change 'authdata_read' output

target/riscv: Change 'authdata_read' output

Use a constant output length and remove the line break to make the
authentication data easier to parse.

Change-Id: Iebbf1f171947ef89b0f360a2cb286a4ea15c6ba5
Signed-off-by: Marc Schink <dev@zapb.de>
Reviewed-on: http://openocd.zylin.com/6199
Tested-by: jenkins
Reviewed-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-by: Tim Newsome <tim@sifive.com>