pinctrl: stmfx: update the result type of dm_i2c_reg_read

Use int as result of dm_i2c_reg_read to avoid warning with
W=1 (warning: comparison is always false due to limited range
of data type [-Wtype-limits])

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@st.com>