net: ipa: tighten up IPA register validity checking

When checking the validity of an IPA register ID, compare it against
all possible ipa_reg_id values.

Rename the function ipa_reg_id_valid() to be specific about what's
being checked.

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