arm: mach-k3: Fix config check for FS_LOADER

arm: mach-k3: Fix config check for FS_LOADER

load_firmware() API calls fs-loader APIs and checks for CONFIG_FS_LOADER
before calling those APIs. The if check only checks for CONFIG_FS_LOADER
but not for CONFIG_SPL_FS_LOADER.

When CONFIG_FS_LOADER is enabled, load_firmware() API calls fs-loader APIs
but this is done at SPL stage and at this time FS_LOADER is not built yet
as a result we see below build error.

  AR      spl/boot/built-in.o
  LD      spl/u-boot-spl
arm-none-linux-gnueabihf-ld.bfd: arch/arm/mach-k3/common.o: in function
`load_firmware':
/home/danish/workspace/u-boot/arch/arm/mach-k3/common.c:184: undefined
reference to `get_fs_loader'
arm-none-linux-gnueabihf-ld.bfd:
/home/danish/workspace/u-boot/arch/arm/mach-k3/common.c:185: undefined
reference to `request_firmware_into_buf'
make[2]: *** [/home/danish/workspace/u-boot/scripts/Makefile.spl:527:
spl/u-boot-spl] Error 1
make[1]: *** [/home/danish/workspace/u-boot/Makefile:2055:
spl/u-boot-spl] Error 2
make[1]: Leaving directory '/home/danish/uboot_images/am64x/r5'
make: *** [Makefile:177: sub-make] Error 2

Fix this by modifying the if check to CONFIG_IS_ENABLED(FS_LOADER) instead
of IS_ENABLED(CONFIG_FS_LOADER) as the former will check for the
appropriate config option (CONFIG_SPL_FS_LOADER / CONFIG_FS_LOADER) based
on the build stage.

Signed-off-by: MD Danish Anwar <danishanwar@ti.com>
Acked-by: Ravi Gunasekaran <r-gunasekaran@ti.com>