fwk: Replace remaining `assert()` call with `fwk_assert()`

A previous commit (7b01ad84) left behind one replacement from
assert() to fwk_assert().

This commit fixes that.

Change-Id: Id98d2d09bb25d2842beeff69b7938fe259d5938c
Signed-off-by: Nicola Mazzucato <nicola.mazzucato@arm.com>