stm32mp: stm32prog: handle the next phase after USB re-enumeration

Handle the second USB enumeration only when the flashlayout is received
and when phase is PHASE_FLASHLAYOUT. This patch removes the call of
stm32prog_next_phase as it is already done in stm32prog_dfu_init().

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