spl: sata: Rework the loading case it not use IS_ENABLED(...)

spl: sata: Rework the loading case it not use IS_ENABLED(...)

In this case, using IS_ENABLED(...) to attempt to load the image results
in harder to read and less useful code, along with having to define a
CONFIG value that would be unused. To maintain the current albeit
slightly odd behavior, maintain that if we have both SPL_FS_FAT and
SPL_SATA_RAW_U_BOOT_USE_SECTOR enabled, we use SPL_FS_FAT for the load.

Signed-off-by: Tom Rini <trini@konsulko.com>