mailbox: bcm-pdc: PDC driver leaves debugfs files after removal

Minor fix to ensure that debugfs stats pseudo-files are
removed when driver module is unloaded.  Previously, the call to
debugfs_remove_recursive() was never being called since the
directory was not empty, and a seg fault would occur if another
process tried to access these leftover files.

Signed-off-by: Steve Lin <steven.lin1@broadcom.com>
Signed-off-by: Rob Rice <rob.rice@broadcom.com>
Reviewed-by: Andy Gospodarek <gospo@broadcom.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>