mtd: nand: pxa3xx: fix raw read when last_chunk_size == 0

Commit 6293b036 ("mtd: nand: pxa3xx: add raw read support") added the
local data_len variable in handle_data_pio() to track read size, but
forgot to update the condition of drain_fifo() call. That happens to
work when the layout last_chunk_size != 0. But when last_chunk_size ==
0, drain_fifo() is not called to read the last chunk, which leads to
"Wait timeout!!!" error. Fix this.

Fixes: 6293b036 ("mtd: nand: pxa3xx: add raw read support")
Cc: Miquel Raynal <miquel.raynal@bootlin.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>