mtd: rawnand: stm32_fmc2: use FMC2_TIMEOUT_MS for timeouts

This patch removes the constant FMC2_TIMEOUT_US.
FMC2_TIMEOUT_MS will be used each time that we need to wait (except
when the timeout value is set by the framework).

It was seen, during stress tests with the sequencer 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>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/1588756279-17289-4-git-send-email-christophe.kerello@st.com