ARM: omap2plus_defconfig: Fix warning due invalid RXKAD symbol value

Commit 648af7fc ("rxrpc: Absorb the rxkad security module") changed
the RXKAD Kconfig symbol from tristate to boolean but the commit didn't
update the omap2plus_defconfig that was enabling CONFIG_RXKAD as module.

This leads to the following warning when using the omap2plus_defconfig:

arch/arm/configs/omap2plus_defconfig:112:warning: symbol value 'm' invalid for RXKAD

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>