staging: vchiq_arm: fix error codes in probe

If vchiq_debugfs_init() fails, then we accidentally return a valid
pointer casted to int on error.  This code is simpler if we get rid of
the "ptr_err" variable and just use "err" throughout.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>