stm32mp: stm32prog: add timeout in stm32prog_serial_get_buffer

Handle timeout in stm32prog_serial_get_buffer to sent NACK
to STM32CubeProgrammer when the buffer is not fully received.

This patch avoids to reach the STM32CubeProgrammer timeout and
the associated unrecoverable error.

  Timeout error occurred while waiting for acknowledgment.

  Error: Write Operation fails at packet number 4165 at address 0x1044FF

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