isdn: icn: buffer overflow in icn_command()

This buffer over was detected using static analysis:

	drivers/isdn/icn/icn.c:1325 icn_command()
	error: format string overflow. buf_size: 60 length: 98

The calculation for the length of the string is off because it assumes
that the dial[] buffer holds a 50 character string, but actually it is
at most 31 characters and NUL.  I have removed the dial[] buffer because
it isn't needed.

The maximum length of the string is actually 79 characters and a NUL.  I
have made the cbuf[] array large enough to hold it and changed the
sprintf() to an snprintf() as a further safety enhancement.

Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>