arm: stm32mp: stm32prog: always flush DFU on start command for uart

Remove the test on data->dfu_seq, because dfu_seq=0 not only when
the DFU is not started (mask with 0xffff). This flush is mandatory
as the final treatment, common with USB, is done in DFU callback.

This patch avoids issue if the received length is a multiple of
the DFU packet.

For example if size of bootfs partition is egual to 0x4000000,
data->dfu_seq=0 at the end of the partition, the flush it not
requested and the phase is not increased in the callback.
U-Boot continue to request the bootfs in the next GetPhase command.

Fixes: 468f0508 ("stm32mp: stm32prog: add serial link support")
Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>