stm32mp: stm32prog: handle interruption during the first enumeration

When an interruption is received during the first USB enumeration
used to received the FlashLayout, with handle ctrl-c, the second
enumeration is not needed and the result for stm32prog_usb_loop
is false (reset is not needed).

This patch avoids the need of a second ctrl to interrupt the command
stm32prog.

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