mtd: docg3 increase write/erase timeout

After several tries with ubifs, it appears empirically that constructor
provided figures for erase/write timeouts are underestimated. A timeout
of 100ms seems to work with a 5 years worn chip, and no timeouts occur
anymore.

Signed-off-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>