[NET]: improve readability of dev_set_promiscuity() in net/core/dev.c

A trivial patch to improve the readability of dev_set_promiscuity()
in net/core/dev.c. New code does exactly the same thing as original
code.

Signed-off-by: David Chau <ddcc@mit.edu>
Signed-off-by: Domen Puncer <domen@coderock.org>
Signed-off-by: David S. Miller <davem@davemloft.net>