Fix waveshare_rp2040_zero USB unrecognize after unplug and replug (#1421

)

Some RP2040-Zero boards from WaveShare can only be recognized via USB
after flashing UF2 into it, and if you unplug and replug it, nothing
happens on both USB host and device, RP2040-Zero seems just not booting.

According to @ArkBrj, it seems that setting `PICO_FLASH_SPI_CLKDIV` to
`2` makes the clock chip exceed the spec, setting it to `4` fixes this
bug.

I tested it with the blink program in pico-examples, this does fix the
bug for me.

Fixes <#1304>.

src/boards/include/boards/waveshare_rp2040_zero.h

@@ -64,15 +64,15 @@
 #define PICO_BOOT_STAGE2_CHOOSE_W25Q080 1
 
 #ifndef PICO_FLASH_SPI_CLKDIV
-#define PICO_FLASH_SPI_CLKDIV 2
+#define PICO_FLASH_SPI_CLKDIV 4
 #endif
 
 #ifndef PICO_FLASH_SIZE_BYTES
 #define PICO_FLASH_SIZE_BYTES (2 * 1024 * 1024)
 #endif
 
 // All boards have B1 RP2040
-#ifndef PICO_RP2040_B0_SUPPORTED 
+#ifndef PICO_RP2040_B0_SUPPORTED
 #define PICO_RP2040_B0_SUPPORTED  0
 #endif