[MTD] Fix default timeouts for Intel NOR flash

In commit c172471b

 Nico switched to using
common code for polling for command completion. Unfortunately he also used
a common default timeout for both write and erase commands, despite the
fact that erases can take a _whole_ lot longer. Use a more sensible
default for erase timeout.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>