rtw88: remove redundant assignment to pointer debugfs_topdir

Pointer debugfs_topdir is initialized to a value that is never read
and it is re-assigned later. The initialization is redundant and can
be removed.

Addresses-Coverity: ("Unused value")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>