net: ipa: rename db_enable flag

In several places, a Boolean flag is used in the GSI code to
indicate whether the "doorbell engine" should be enabled or not
when a channel is configured.  This is basically done to abstract
this property from the IPA version; the GSI code doesn't otherwise
"know" what the IPA hardware version is.  The doorbell engine is
enabled only for IPA v3.5.1, not for IPA v4.0 and later.

The next patch makes another change that affects behavior during
channel reset (which also involves programming the channel).  It
also distinguishes IPA v3.5.1 hardware from newer hardware.

Rather than creating another flag whose value matches the "db_enable"
value, just rename "db_enable" to be "legacy" so it can be used to
signal more than just the special doorbell handling.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>