drivers/spi: Sanitize headers from preprocessor abuse

Continuing on from the rational given in:

a173a62 Remove guarding #includes by CONFIG_FOO combinations

Change-Id: I35c636ee7c0b106323b3e4b90629f7262750f8bd
Signed-off-by: Edward O'Callaghan <eocallaghan@alterapraxis.com>
Reviewed-on: http://review.coreboot.org/6114
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <patrick@georgi-clan.de>