chan_dahdi.c: Resolve a format-truncation build warning.

With gcc (Ubuntu 11.2.0-19ubuntu1) 11.2.0:

> chan_dahdi.c:4129:18: error: ‘%s’ directive output may be truncated
>   writing up to 255 bytes into a region of size between 242 and 252
>   [-Werror=format-truncation=]

This removes the error-prone sizeof(...) calculations in favor of just
doubling the size of the base buffer.

Change-Id: I2d276785286730d3d5d0a921bcea2e065dbf27c5