drivers/net/wireless/hostap: Integer overflow

The local variable 'value' comes from 'extra', a parameter of function
'prism2_ioctl_priv_prism2_param'. If a large number passed to 'value',
there would be an integer overflow in the following line:
	local->passive_scan_timer.expires = jiffies +
		local->passive_scan_interval * HZ

Signed-off-by: Wenliang Fan <fanwlexca@gmail.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>