lib: arm_ffa: Downgrade our FF-A version if lower

The SPMC returns the version it supports as the
return value of FFA_VERSION. If ours is lower than
that but valid, downgrade our ffa_version variable
instead of using the one reported by the SPMC.

Bug: None
Change-Id: Ic65bbb13b7cbadd113b9757d37b3c90ceb8e1462