cmd: pinmux: Fix warnings when compiling with W=1

This patch solves the following warnings:

cmd/pinmux.c: In function 'do_dev':
cmd/pinmux.c:26:6: warning: this statement may fall through [-Wimplicit-fallthrough=]
   if (ret) {
      ^
cmd/pinmux.c:30:2: note: here
  case 1:
  ^~~~

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>