package/w_scan: needs headers >= 3.7

w_scan uses FEC_2_5, introduced in Linux 3.7.

Also fix the comment, that was stuck to headers 3.0 when we were
requesting headers >= 3.3.

Fixes:
    http://autobuild.buildroot.org/results/bb9/bb97bff1bf7774ebd43ba2107d84520538e530cf/
    http://autobuild.buildroot.org/results/9c2/9c28dcaf3d3bb65977d565e4fbbc25d51d45ac41/


    ...

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>