Workaround for "noise" problems with app_rxfax in current LCR.

Workaround for "noise" problems with app_rxfax in current LCR.

The problem: for some reason, asterisk silently switches write_format
from SLINEAR to ALAW when using app_rxfax, thereby effectively disabling
automatic conversion SLINEAR->ALAW and thereby breaking app_rxfax after
the first call to ast_write(). Read: instantly, after the first data frame.

Older versions of LCR work flawlessly, until we find out, what really triggers
the problem, we at least switch the channel back, since it never makes
sense to send something different than ALAW to chan_lcr.