[EFM32] Remove erroneous sleep() call from blocking spi_master_write.

libraries/mbed/targets/hal/TARGET_Silicon_Labs/TARGET_EFM32/spi_api.c

@@ -359,7 +359,6 @@ int spi_master_write(spi_t *obj, int value)
 
     /* Wait for transmission of last byte */
     while (!(obj->spi.spi->STATUS & USART_STATUS_TXC)) {
-        sleep(); // TODO_LP this might break other code, write should be separate from read?
     }
 
     return spi_read(obj);