staging: rtl8188eu: replace NULL comparison with variable

Replace NULL comparison of the variable with the variable name or
!variable to conform to the Linux kernel coding style. Issue found using
checkpatch.

Signed-off-by: Santha Meena Ramamoorthy <santhameena13@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>