HSI: omap_ssi: Fix return value check in ssi_debug_add_ctrl()

In case of error, the function debugfs_create_*() returns NULL
pointer not ERR_PTR() if debugfs is enabled. The IS_ERR() test
in the return value check should be replaced with NULL test.

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
Signed-off-by: Sebastian Reichel <sre@kernel.org>