pc-bios/s390-ccw/net: Update code for the latest changes in SLOF

pc-bios/s390-ccw/net: Update code for the latest changes in SLOF

The ip_version information now has to be stored in the filename_ip_t
structure, and there is now a common function called tftp_get_error_info()
which can be used to get the error string for a TFTP error code.
We can also get rid of some superfluous "(char *)" casts now.

Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
Tested-by: Viktor Mihajlovski <mihajlov@linux.vnet.ibm.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>