Fix flash driver for proper SIO ops

Set SPI peripheral to SIO mode for all operations except READ.
Set back SPI Peripheral to DIO after said operation.

rtl837x_flash.h

@@ -13,7 +13,6 @@
 
 void flash_init(uint8_t enable_dio);
 void flash_read_uid(void);
-void flash_write_enable(void);
 void flash_dump(uint8_t len);
 void flash_read_jedecid(void);
 void flash_read_security(void);