staging: rtl8192u: r8192U_dm: Fix space issue around operators.

Add spaces around the operators.
Issue found by checkpatch.pl

CHECK: spaces preferred around that '+'
CHECK: spaces preferred around that '*'

Signed-off-by: Sanjana Sanikommu <sanjana99reddy99@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>