staging:rtl8192u: Remove unused DM_check_fsync_time_interval - Style

Remove the unused definition for DM_check_fsync_time_interval.

This is a 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>