mtd: m25p80: Use SPI_MEM_OP_NO_DUMMY instead of SPI_MEM_OP_DUMMY(0, x)

SPI_MEM_OP_DUMMY(0, x) means there's 0 dummy bytes to send, which is
similar to SPI_MEM_OP_NO_DUMMY except it's less clear. Use
SPI_MEM_OP_NO_DUMMY instead of SPI_MEM_OP_DUMMY(0, x) in
m25p80_write().
Also stop updating op.dummy.buswidth since this value is only
meaningful if you have dummy bytes.

Reported-by: Cyrille Pitchen <cyrille.pitchen@microchip.com>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Acked-by: Marek Vasut <marek.vasut@gmail.com>