spi: dt-bindings: spi-controller: Fix spi-[rt]x-bus-width for slave mode

spi: dt-bindings: spi-controller: Fix spi-[rt]x-bus-width for slave mode

The descriptions for the spi-rx-bus-width and spi-tx-bus-width
properties refer to "MISO" and "MOSI", which are not explained in the
document.  While these abbreviations are fairly common when talking
about SPI, and thus may not need an explanation, they are not entirely
correct in this context, as the SPI controller may be used in slave mode
instead of master mode.

Fix this by replacing them by "read transfers" resp. "write transfers",
like is done for the spi-rx-delay-us and spi-tx-delay-us properties.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200306085038.8111-3-geert+renesas@glider.be
Signed-off-by: Mark Brown <broonie@kernel.org>