mtd: nand: Support 'EXIT GET STATUS' command in nand_command[_lp]()

READ0 is sometimes used to exit GET STATUS mode. When this is the case
no address cycles are requested, and we can use this information to
detect that READSTART should not be issued after READ0 or that we
shouldn't wait for the chip to be ready.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Tested-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>