powerpc: mpc85xx: Do not build errata command in SPL

The errata command is useless in SPL, so don't build it. This fixes
multiple build failures on PowerPC.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: York Sun <york.sun@nxp.com>
Fixes: 92623672 ("fsl: usb: make errata function common for PPC and ARM")