spi: microchip-core-qspi: don't attempt to transmit during emulated read-only dual/quad operations

In the Linux kernel, the following vulnerability has been resolved:

spi: microchip-core-qspi: don't attempt to transmit during emulated read-only dual/quad operations

The core will deal with reads by creating clock cycles itself, there's

no need to generate clock cycles by transmitting garbage data at the

driver level. Further, transmitting garbage data just bricks the transfer

since QSPI doesn't have a dedicated master-out line like MOSI in regular

SPI. I'm not entirely sure if the transfer is bricked because of the

garbage data being transmitted on the bus or because the core loses

track of whether it is supposed to be sending or receiving data.