spi: stm32h7: rework rx fifo read function

Remove flush parameter and check RXWNE or RXPLVL when end of transfer
flag is set.

Signed-off-by: Amelie Delaunay <amelie.delaunay@foss.st.com>
Signed-off-by: Alain Volmat <alain.volmat@foss.st.com>
Link: https://lore.kernel.org/r/1625646426-5826-6-git-send-email-alain.volmat@foss.st.com


Signed-off-by: Mark Brown <broonie@kernel.org>