ieee80211: redefine S1G bits with GENMASK

The S1G capability fields were defined by ORing BITS()
together, and expecting a custom macro to use the _SHIFT
definitions. Use the Linux kernel GENMASK for the
definitions now, and FIELD_{GET,PREP} to access the fields
in the future.

Take the chance to rename eg. S1G_CAPAB_B0 to the more
compact S1G_CAP0.

Signed-off-by: Thomas Pedersen <thomas@adapt-ip.com>
Link: https://lore.kernel.org/r/20200908190323.15814-2-thomas@adapt-ip.com
Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>