mtd: spi-nor: move #define SPINOR_OP_WRDI

The write disable (WRDI) opcode is not really specific to the SST flashes
(anymore?) -- move the #define to the main opcode group, just before WREN.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>