mtd: spi-nor: fix DMA unsafe buffer issue in spi_nor_read_sfdp()

spi_nor_read_sfdp() calls nor->read() to read the SFDP data.
When the m25p80 driver is used (pretty common case), nor->read() is then
implemented by the m25p80_read() function, which is likely to initialize a
'struct spi_transfer' from its buf argument before appending this
structure inside the 'struct spi_message' argument of spi_sync().

Besides the SPI sub-system states that both .tx_buf and .rx_buf members of
'struct spi_transfer' must point into dma-safe memory. However, two of the
three calls of spi_nor_read_sfdp() were given pointers to stack allocated
memory as buf argument, hence not in a dma-safe area.
Hopefully, the third and last call of spi_nor_read_sfdp() was already
given a kmalloc'ed buffer argument, hence dma-safe.

So this patch fixes this issue by introducing a
spi_nor_read_sfdp_dma_unsafe() function which simply wraps the existing
spi_nor_read_sfdp() function and uses some kmalloc'ed memory as a bounce
buffer.

Fixes: f384b352

 ("mtd: spi-nor: parse Serial Flash Discoverable Parameters (SFDP) tables")
Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Cyrille Pitchen <cyrille.pitchen@wedev4u.fr>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>