Staging: rtl8188eu: core: Use !x in place of NULL comparisons

Change (x == NULL) to !x and (x != NULL) to x, to fix
following checkpatch.pl warnings:
CHECK: Comparison to NULL could be written "!x".

Signed-off-by: Puranjay Mohan <puranjay12@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>