In SPI slave case the transceive should return either the negative
errno code in case of error or the number of frames received.
So, now:
1. the spi_stm32_get_err() routine already checks whether
the SPI cell got an error and returns -EIO in that case.
2. the transceive() routine always returns whatever the
spi_context_wait_for_completion() has returned, which
is either:
a. -EIO in case of error
b. 0 in spi_master ok case
c. the number of frames received in spi_slave ok case
Signed-off-by: Armando Visconti <armando.visconti@st.com>
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