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: Adam Ford <aford173@gmail.com>