Remove broken assertion in console_putc()

The assertion in console_putc() would trigger a recursion that
exhausts the stack and eventually aborts.

This patch replaces the assertion with an error return if the
console has not been initialized yet.

Fixes ARM-software/tf-issues#208

Change-Id: I95f736ff215d69655eb5ba7ceac70dc1409d986a
Merge pull request #142 from athoelke/at/fix-console_putc

Remove broken assertion in console_putc()