semihosting: create file in smh_fs_write_at()

If a file does not exist, it should be created.

Fixes: f676b45151c3 ("fs: Add semihosting filesystem")
Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
diff --git a/fs/semihostingfs.c b/fs/semihostingfs.c
index 96eb334..3592338 100644
--- a/fs/semihostingfs.c
+++ b/fs/semihostingfs.c
@@ -57,8 +57,12 @@
 {
 	long fd, size, ret;
 
+	/* Try to open existing file */
 	fd = smh_open(filename, MODE_READ | MODE_BINARY | MODE_PLUS);
 	if (fd < 0)
+		/* Create new file */
+		fd = smh_open(filename, MODE_WRITE | MODE_BINARY);
+	if (fd < 0)
 		return fd;
 	ret = smh_seek(fd, pos);
 	if (ret < 0) {