virt/coco/sev-guest: Convert the sw_exit_info_2 checking to a switch-case

snp_issue_guest_request() checks the value returned by the hypervisor in
sw_exit_info_2 and returns a different error depending on it.

Convert those checks into a switch-case to make it more readable when
more error values are going to be checked in the future.

No functional changes.
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Link: https://lore.kernel.org/r/20230307192449.24732-8-bp@alien8.de

arch/x86/kernel/sev.c

@@ -2210,15 +2210,21 @@ int snp_issue_guest_request(u64 exit_code, struct snp_req_data *input, unsigned
 		goto e_put;
 
 	*fw_err = ghcb->save.sw_exit_info_2;
-	if (ghcb->save.sw_exit_info_2) {
+	switch (*fw_err) {
+	case 0:
+		break;
+
+	case SNP_GUEST_REQ_INVALID_LEN:
 		/* Number of expected pages are returned in RBX */
-		if (exit_code == SVM_VMGEXIT_EXT_GUEST_REQUEST &&
-		    ghcb->save.sw_exit_info_2 == SNP_GUEST_REQ_INVALID_LEN) {
+		if (exit_code == SVM_VMGEXIT_EXT_GUEST_REQUEST) {
 			input->data_npages = ghcb_get_rbx(ghcb);
 			ret = -ENOSPC;
-		} else {
-			ret = -EIO;
+			break;
 		}
+		fallthrough;
+	default:
+		ret = -EIO;
+		break;
 	}
 
 e_put: