If a SIR dongle is built in the kernel while IRTTY_SIR is built
as a module, kernel compilation will fail.
Thus, the SIR dongle config should depend on the IRTTY_SIR.

Closes kernel bug# 6512
(http://bugzilla.kernel.org/show_bug.cgi?id=6512)
Signed-off-by: Samuel Ortiz <samuel@sortiz.org>
Signed-off-by: David S. Miller <davem@davemloft.net>