drivers: spi: stm32h7: Use transferSize and EOT

Set the transfer size in SPI H7 and check EOT instead of TXC
to be sure the transaction has finished. This is required to
enable the use of the SPI FIFO, as otherwise SPI seems to
operate in "continuous mode", which produces several SCK cycles
after the last frame has been sent/received. More details in the PR.

Signed-off-by: Daniel Gaston Ochoa <dgastonochoa@gmail.com>