rt2870sta: Use latest firmware for RT3070 and RT3071

rt2870sta: Use latest firmware for RT3070 and RT3071

Ralink's original drivers for RT2800U-family chips used multiple
different versions of the firmware for different chips.  The rt2870sta
driver in staging was briefly converted to load different files for
different chips.  However, the current rt2870.bin provides 2 images
that are supposed to cover all of them, so:

- Replace rt3070.bin with a symlink.  The driver will use the first
  image and ignore the second.
- Replace rt3071.bin with a copy of the second image in the current
  rt2870.bin.

Signed-off-by: Ben Hutchings <ben@decadent.org.uk>