mtd: m25p80 timeout too short for worst-case m25p16 devices

The m25p16 data sheet from numonyx lists the worst-case bulk erase time
(tBE) as 40 seconds.

Signed-off-by: Steven A. Falco <sfalco@harris.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>