staging: wlan-ng: add parentheses to macro argument usage in prism2mgmt.c

Fix two "CHECK: Macro argument 'N' may be better as '(N)' to avoid
precedence issue" messages, reported by checkpatch, by adding
parentheses around the offending macro argument references.

Signed-off-by: Tim Collier <osdevtc@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>