res_fax: Fix latent bug exposed by ASTERISK-24841 changes.

Three fax related tests started failing as a result of changes made for
ASTERISK-24841:
tests/fax/pjsip/gateway_t38_g711
tests/fax/sip/gateway_mix1
tests/fax/sip/gateway_mix3

Historically, ast_channel_make_compatible() did nothing if the channels
were already "compatible" even if they had a sub-optimal translation path
already setup.  With the changes from ASTERISK-24841 this is no longer
true in order to allow the best translation paths to always be picked.  In
res_fax.c:fax_gateway_framehook() code manually setup the channels to go
through slin and then called ast_channel_make_compatible().  With the
previous version of ast_channel_make_compatible() this was always a
no-operation.

* Remove call to ast_channel_make_compatible() in fax_gateway_framehook()
that now undoes what was just setup when the framehook is attached.

* Fixed locking around saving the channel formats in
fax_gateway_framehook() to ensure that the formats that are saved are
consistent.

* Fix copy pasta errors in fax_gateway_framehook() that confuses read and
write when dealing with saved channel formats.

ASTERISK-24841
Reported by: Matt Jordan

Change-Id: I6fda0877104a370af586a5e8cf9e161a484da78d