flash: at91samd: fix use of is_erased in check

is_erased can be one of -1, 0, or 1 so it must not be checked like a
boolean value.  In this case we want to erase a page unless we know it's
already erased so we just check for is_erased != 1.

Thanks to Jim Paris for pointing this out on another driver.

Change-Id: I4591186228153b64e5a9608a2aac18745e578d4a
Signed-off-by: Andrey Yurovsky <yurovsky@gmail.com>
Reviewed-on: http://openocd.zylin.com/2368
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>