target/riscv: fix dead assignment

target/riscv: fix dead assignment

Scan-build reports:
	Unused code: Dead nested assignment
	riscv.c:459 Although the value stored to 'ir_user4_raw' is
		used in the enclosing expression, the value is
		never actually read from 'ir_user4_raw'

This is caused by the value reassigned in 'ir_user4_raw':
	riscv.c:459 ir_user4[3] = (uint8_t)(ir_user4_raw >>= 8);
but never used.

Drop the DIY conversion in favor of h_u32_to_le() that does not
reassign the input value.

Change-Id: Ifad29f4c46d4a2d0a2f5a5c4104d768cc3db2794
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7208
Reviewed-by: Tim Newsome <tim@sifive.com>
Reviewed-by: Jan Matyas <matyas@codasip.com>
Tested-by: jenkins