Staging: wlan-ng: fix sparse warning in prism2fw.c

Fix the following sparse warning :

In file included from drivers/staging/wlan-ng/prism2usb.c:5:0:
drivers/staging/wlan-ng/prism2fw.c: In function
‘read_cardpda.constprop.43’:
drivers/staging/wlan-ng/prism2fw.c:792:1: warning: the frame size of
1068 bytes is larger than 1024 bytes [-Wframe-larger-than=]

The variable to 'struct p80211msg_p2req_readpda' was previously being created
on the stack, which inturn exeeded the frame size limit, resulting in a
sparse warning. This patch alloctes the memory to the structure dynamically
and the operations are left unchanged.

Signed-off-by: A Raghavendra Rao <arrao@cdac.in>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>