firmware: ti_sci: Fix an off-by-one in ti_sci_debugfs_create()

The ending NULL is not taken into account by strncat(), so switch to
snprintf() to correctly build 'debug_name'.

Using snprintf() also makes the code more readable.

Fixes: aa276781

 ("firmware: Add basic support for TI System Control Interface (TI-SCI) protocol")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://lore.kernel.org/r/7158db0a4d7b19855ddd542ec61b666973aad8dc.1698660720.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Nishanth Menon <nm@ti.com>