The current spi_read_buf function fails on SPI host masters which
are only half-duplex capable. Splitting the Tx and Rx part solves
this issue.

Tested on Raspberry Pi (full duplex) and I2SE Duckbill (half duplex).
Signed-off-by: Michael Heimpold <mhei@heimpold.de>
Signed-off-by: David S. Miller <davem@davemloft.net>