Staging: rtl8192u: Clean up tests if NULL returned on failure

Some functions return 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 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>