stm32mp: stm32prog: remove all the header check for UART download

This patch removes the header check for UART download;
the check of checksum is not mandatory with even parity and chuck
checksum for each 256 received bytes and it is only done for
STM32 image (FSBL = TF-A BL2), not for FIT image.

This patch solve issue of duplicated 0x100 byte written with FIP header.

Fixes: 4fb7b3e1 ("stm32mp: stm32prog: add FIP header support")
Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>