spi: cadence: Ensure data lines set to low during dummy-cycle period

During dummy-cycles xSPI will switch GPIO into Hi-Z mode. In that dummy
period voltage on data lines will slowly drop, what can cause
unintentional modebyte transmission. Value send to SPI memory chip will
depend on last address, and clock frequency.
To prevent unforeseen consequences of that behaviour, force send
single modebyte(0x00).
Modebyte will be send only if number of dummy-cycles is not equal
to 0. Code must also reduce dummycycle byte count by one - as one byte
is send as modebyte.

Signed-off-by: Witold Sadowski <wsadowski@marvell.com>
Link: https://msgid.link/r/20240529074037.1345882-2-wsadowski@marvell.com
Signed-off-by: Mark Brown <broonie@kernel.org>