mtd: cfi_cmdset_0002: Fix do_erase_chip() to get chip as erasing mode

The chip state is set to erasing by the function after getting chip.
So it should be to get chip as erasing mode at first.
But previously it was to get chip as writing mode then fix as erasing.

Signed-off-by: Tokunori Ikegami <ikegami.t@gmail.com>
Cc: linux-mtd@lists.infradead.org
Signed-off-by: Richard Weinberger <richard@nod.at>