staging: wlan-ng: Replace data type declaration with variable of same type in prism2mgmt.c

sizeof(var) instead of sizeof(struct XXX) is preferred.

It also fix the following checkpatch.pl script warning:
WARNING: line over 80 characters

Fix this two in prism2mgmt.c file.

Signed-off-by: Sergio Paracuellos <sergio.paracuellos@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>