ARM: da850evm: Fix broken SPI Flash

A previous patch converted a bunch of settings for CONFIG_SF_DEFAULT
but it broke the SPI Flash which now prevents booting.  This patch
now correctly sets CONFIG_SF_DEFAULT_SPEED=30000000 to match what it
was before the conversion.

Fixes: 14453fbf ("Convert CONFIG_SF_DEFAULT_* to Kconfig")
Signed-off-by: NAdam Ford <aford173@gmail.com>

configs/da850evm_defconfig

@@ -55,6 +55,7 @@ CONFIG_DM_MMC=y
 CONFIG_MTD_DEVICE=y
 CONFIG_DM_SPI_FLASH=y
 CONFIG_SPI_FLASH=y
+CONFIG_SF_DEFAULT_SPEED=30000000
 CONFIG_SPI_FLASH_STMICRO=y
 CONFIG_SPI_FLASH_WINBOND=y
 CONFIG_SPI_FLASH_MTD=y