gr_rasta_adcdac.c: Unchecked return value from library error spotted by Coverity

[Ryan Long]

CID 1399780: Unchecked return value from library in gr_rasta_adcdac_init1().

396        strcpy(priv->prefix, "/dev/rastaadcdac0");
397        priv->prefix[16] += dev->minor_drv;
   CID 1399780 (#1 of 1): Unchecked return value from library (CHECKED_RETURN)2. check_return: Calling mkdir(priv->prefix, 511U) without checking return value. This library function may fail and return an error code. [Note: The source code implementation of the function has been overridden by a builtin model.]
398        mkdir(priv->prefix, S_IRWXU | S_IRWXG | S_IRWXO);
399        priv->prefix[17] = '/';
400        priv->prefix[18] = '\0';

[Ryan Long <ryan.long@…>]

In 91e38f8/rtems:

gr_rasta_adcdac.c: Fix Unchecked return value from library (CID #1399780)

CID 1399780: Unchecked return value from library in gr_rasta_adcdac_init1().

Closes #4287