cxgb3: avoid needless buffer copy for firmware

There's no reason to perform a buffer copy for the firmware name. This
also avoids a (currently impossible with current callers) NULL dereference
if there was no matching firmware.

Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: David S. Miller <davem@davemloft.net>