cc2538: spi: Do not call SPI_WAITFOREORx() at end of init

If the SSI has never been used and spi_init() is called, then the SSI receive
FIFO is empty and remains so, so calling SPI_WAITFOREORx() at the end of
spi_init() waits endlessly for SSI_SR.RNE to be set. Hence, this call must be
removed in order to avoid a deadlock.

Signed-off-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>

cpu/cc2538/dev/spi.c

@@ -91,8 +91,5 @@ spi_init(void)
 
   /* Enable the SSI */
   REG(SSI0_BASE + SSI_CR1) |= SSI_CR1_SSE;
-
-  /* Clear the RX FIFO */
-  SPI_WAITFOREORx();
 }
 /** @} */