net: sockets: Fix mask application in sock_set_flag()

We pass normal 1-based mask, and mask invert it before logically
AND'ing with the value. This apparently a mix-up between how the
mask was intended to be passed initially and how it was in the end.
This issue actually didn't have an effect, because currently defined
flags have mutually exclusive lifetime (when "eof" flag is set,
"non-blocking" flag value is no longer import). Anyway, that's a
bug and needs fixing.

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>