man polling
...
SUPPORTED DEVICES
Device polling requires explicit modifications to the device drivers. As
of this writing, the dc(4), em(4), fwe(4), fwip(4), fxp(4), ixgb(4),
nge(4), re(4), rl(4), sf(4), sis(4), ste(4), vge(4), vr(4), and xl(4)
devices are supported, with others in the works.

がーん、cobra2247 (Cobra)の I/F って bge だから、この機能ないのね。
/usr/src/sys/dev/bge で grep POLLING しても、何も出てこないし。