drivers: flash: stm32 xspi: early init exit for memmap mode

Exit the init earlier when XSPI is in memory map mode. Avoid
unnecessary checks and prevent pin reconfiguration that might
cause line spikes. Clock check beforehand is preserved.

Remove '\n' from the LOG_DBG string.

Signed-off-by: Georgij Cernysiov <geo.cgv@gmail.com>