drivers: stm32: i2c rtio: Fix 'timings' array size

'timings' is an array of 'struct i2c_config_timing' (3 x uint32_t).
'i2c_timings_##index' is an array of uint32_t (hence the cast when it
is assigned to 'timings'). Therefore 'ARRAY_SIZE(i2c_timings_##index)' is
off by a factor 3 when used for n_timings.

Signed-off-by: Abderrahmane JARMOUNI <git@jarmouni.me>