wlan-ng/prism2mgmt.c: added parantheses to macro

Enclose the macro into braces so that it can be closed by a semicolon

Signed-off-by: Sebastian Wankerl <sisewank@cip.cs.fau.de>
Signed-off-by: Sebastian Ehrenfels <qi50dube@cip.cs.fau.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>