s390/hmcdrv: free memory on error path

Free allocated page in case of error returned by hmcdrv_ftp_startup.

[heiko.carstens@de.ibm.com]: slightly changed Christophe's patch

Signed-off-by: Christophe Jaillet <christophe.jaillet@wanadoo.fr>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>