/**
 * @brief Transfer a number of bytes from/to a given register address
 *
 * This function is a shortcut function for easier handling of register based SPI devices. As
 * many SPI devices use a register based addressing scheme, this function is a convenient short-
 * cut for interfacing with such devices.
 *
 * @param[in] dev       SPI device to use
 * @param[in] reg       Register address to transfer data to/from
 * @param[in] out       Byte array to send data from, set NULL if only receiving
 * @param[out] in       Byte buffer to read into, set NULL if only sending
 * @param[in] length    Number of bytes to transfer
 *
 * @return              Number of bytes that were transfered
 * @return              -1 on error
 */
int spi_transfer_regs(spi_t dev, uint8_t reg, char *out, char *in, unsigned int length);