Staging: rtl8712: Clean up tests if NULL returned on failure

Some functions like kmalloc/usb_alloc_urb/kmalloc_array 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>