mtd: rawnand: Rename the use_bufpoi variables

Both in nand_do_read_ops() and nand_do_write_ops() there is a boolean
called use_bufpoi which is set to true in case of unaligned request or
when there is a need for a DMA-able buffer. It basically means "use a
bounce buffer".

Depending on the value of use_bufpoi, the bufpoi variable is always
used and will either point to the original buffer or to the nand_chip
structure "internal data buffer" (this buffer is allocated with
kmalloc() on purpose so that it will be DMA-compliant).

In all cases bufpoi is used so the boolean name is misleading. Rename
use_bufpoi to be use_bouce_buf to be more accurate.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Boris Brezillon <boris.brezillon@collabora.com>
Link: https://lore.kernel.org/linux-mtd/20200507105241.14299-6-miquel.raynal@bootlin.com