mtd: spi-nor: fix silent truncation in spi_nor_read_raw()

spi_nor_read_raw() assigns the result of 'ssize_t spi_nor_read_data()'
to the 'int ret' variable, while 'ssize_t' is a 64-bit type and *int*
is a 32-bit type on the 64-bit machines. This silent truncation isn't
really valid, so fix up the variable's type.

Fixes: f384b352

 ("mtd: spi-nor: parse Serial Flash Discoverable Parameters (SFDP) tables")
Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>