mtd: rawnand: stm32_fmc2: use FMC2_TIMEOUT_5S for timeouts

FMC2_TIMEOUT_5S will be used each time that we need to wait.
It was seen, during stress tests in an overloaded system,
that we could be close to 1 second, even if we never met this
value. To be safe, FMC2_TIMEOUT_MS is set to 5 seconds.

Signed-off-by: Christophe Kerello <christophe.kerello@st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@st.com>