csp_error: Add CSP_ERR_NOSYS for unimplemented functions
This commit adds CSP_ERR_NOSYS with the value 38.  This error number
is taken from POSIX.1-2001 ENOSYS.  Depending on CSP_HAVE_STDIO, some
functions won't be included in libcsp.  In that case, the functions
should return -ENOSYS.
#define CSP_ERR_TX -10 /**< Transmission failed */
#define CSP_ERR_DRIVER -11 /**< Error in driver layer */
#define CSP_ERR_AGAIN -12 /**< Resource temporarily unavailable */
+#define CSP_ERR_NOSYS -38 /**< Function not implemented */
#define CSP_ERR_HMAC -100 /**< HMAC failed */
#define CSP_ERR_CRC32 -102 /**< CRC32 failed */
#define CSP_ERR_SFP -103 /**< SFP protocol error or inconsistency */
