target/riscv: fix use of uninitialized value

target/riscv: fix use of uninitialized value

Scan-build reports:
	Logic error: Uninitialized argument value
	riscv.c:2688 2nd function call argument is an uninitialized value

This is a real error cause by running the command "riscv
authdata_write" without arguments. In such case 'value' is not
initialized and is passed to and used by r->authdata_write().

Reorganize the code to:
- detect the correct amount or command's arguments;
- drop the LOG_ERROR() on ERROR_COMMAND_SYNTAX_ERROR;
- drop the 'else' after 'return'.

Change-Id: I62e031220593b8308bc674b753e15d16d4c5c9ac
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7210
Tested-by: jenkins
Reviewed-by: Jan Matyas <matyas@codasip.com>
Reviewed-by: Tim Newsome <tim@sifive.com>