target/riscv: fix unused initialization

target/riscv: fix unused initialization

Scan-build reports:
	Unused code: Dead assignment
	riscv.c:716 Value stored to 'result' is never read

Remove the initialization of variable 'result'.

Change-Id: Ied67bb4fcfa5bace186522074247ead43a5d5cd5
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7207
Reviewed-by: Tim Newsome <tim@sifive.com>
Reviewed-by: Jan Matyas <matyas@codasip.com>
Tested-by: jenkins