mtd: docg4: fix status polling loop

The loop that polls the status register waiting for an operation to complete
foolishly bases the timeout simply on the number of loop iterations that have
ocurred.  When I increased the processor clock speed, timeouts started to appear
for long block erasure operations.  This patch measures the timeout using
jiffies.

Signed-off-by: Mike Dunn <mikedunn@newsguy.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>