spl: Fix guardian macros in spl.h

Fix the macros guarding the spl.h header for various platforms. Due to
a typo and a propagation of it, the macros went out-of-sync with their
ifdef check, so fix this.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Tom Rini <trini@ti.com>