x86/platform/olpc: Correct ifdef symbol to intended CONFIG_OLPC_XO15_SCI

The refactoring in the commit in Fixes introduced an ifdef
CONFIG_OLPC_XO1_5_SCI, however the config symbol is actually called
"CONFIG_OLPC_XO15_SCI".

Fortunately, ./scripts/checkkconfigsymbols.py warns:

OLPC_XO1_5_SCI
Referencing files: arch/x86/platform/olpc/olpc.c

Correct this ifdef condition to the intended config symbol.

Fixes: ec9964b4 ("Platform: OLPC: Move EC-specific functionality out from x86")
Suggested-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: <stable@vger.kernel.org>
Link: https://lkml.kernel.org/r/20210803113531.30720-3-lukas.bulwahn@gmail.com