riscv: don't write to mstatus.XS

XS is a read-only field of mstatus. Unable to be write. So remove this code.

Change-Id: I3ad6b0029900124ac7cce062e668a0ea5a8b2c0e
Signed-off-by: Xiang Wang <wxjstz@126.com>
Reviewed-on: https://review.coreboot.org/28357
Reviewed-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Reviewed-by: Philipp Hug <philipp@hug.cx>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

src/arch/riscv/virtual_memory.c

@@ -43,7 +43,6 @@ void mstatus_init(void)
 	uintptr_t ms = 0;
 
 	ms = INSERT_FIELD(ms, MSTATUS_FS, 3);
-	ms = INSERT_FIELD(ms, MSTATUS_XS, 3);
 	write_csr(mstatus, ms);
 
 	// clear any pending timer interrupts.