console: add console_has_tstc helper function for CONSOLE_MUX

Add the helper function console_has_tstc() and replace the test
#if CONFIG_IS_ENABLED(CONSOLE_MUX) to a simple 'if' test to
respect the U-Boot coding rule.

No functional change.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
1 file changed