Revert "rose: zero length frame filtering in af_rose.c"

This reverts commit 244f46ae

.

Alan Cox did the research, and just like the other radio protocols
zero-length frames have meaning because at the top level ROSE is
X.25 PLP.

So this zero-length filtering is invalid.

Signed-off-by: David S. Miller <davem@davemloft.net>