console: unify fgetc function when console MUX is deactivated

console: unify fgetc function when console MUX is deactivated

Unify the fgetc function when MUX is activated or not:
- always call tstc() : it is the normal behavior expected
  by serial uclass (call tstc then getc) and that avoids
  issue when SERIAL_RX_BUFFER is activated
- reload WATCHDOG in the char waiting loop

This patch allow to have the same behavior when CONSOLE_MUX is activated
or not and avoid regression when this feature is deactivated.

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