staging:rtl8192u: Replace magic number with defined constant - Style

The defined constant MIMO_PS_STATIC is used for this test for zero
elsewhere in code so the magic number '0' has been replaced with that
comment, which was actually explicitly mentioned in the comment.

This is a simple coding style change which should have no impact on
runtime code execution.

Signed-off-by: John Whitmore <johnfwhitmore@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>