Staging: rtl8723au: Clean up tests if NULL returned on failure

Some functions like kmalloc/usb_alloc_urb returns Null as their return
value on failure. !x is generally preferred over x==NULL or NULL==x. So
make use of !x if the value returned on failure by these functions is NULL.
Done using coccinelle:

@@
expression e;
statement S;
@@
e = \(kmalloc\|devm_kzalloc\|kmalloc_array
     \|devm_ioremap\|usb_alloc_urb\|alloc_netdev\)(...);
- if(e==NULL)
+ if(!e)
  S

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>