stm32mp: update uart number in trace of serial device not found

Align the uart number in the trace of setup_boot_mode() with the name of
the uart/usart device (start at 1) and not with the instance value
(start at 0), i.e. the serial device sequence number and the index in
serial_addr[].

Fixes: f49eb16c ("stm32mp: stm32prog: replace alias by serial
device sequence number")

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