i2c: slave: improve sanity check when registering

[ Upstream commit 1b1be3bf ]

Add check for ERR_PTR and simplify code while here.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Alain Volmat <alain.volmat@st.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>