samd/samd_qspiflash: Correct QSPI baud calculation.

The QSPI baud is derived from the AHB clock, not from the APB (peripheral)
clock.

Datasheet: The QSPI Baud rate clock is generated by dividing the module
clock (CLK_QSPI_AHB) by a value between 1 and 255.

As previously implemented, all baudrates are 2.5 times greater than
expected.

Signed-off-by: I. Tomita <ziceva@me.com>

ports/samd/samd_qspiflash.c

@@ -238,7 +238,7 @@ static void wait_for_flash_ready(void) {
 }
 
 static uint8_t get_baud(int32_t freq_mhz) {
-    int baud = get_peripheral_freq() / (freq_mhz * 1000000) - 1;
+    int baud = get_cpu_freq() / (freq_mhz * 1000000) - 1;
     if (baud < 1) {
         baud = 1;
     }