net: ipa: use IPA register IDs to determine offsets

Expose two inline functions that return the offset for a register
whose ID is provided; one of them takes an additional argument
that's used for registers that are parameterized.  These both use
a common helper function __ipa_reg_offset(), which just uses the
offset symbols already defined.

Replace all references to the offset macros defined for IPA
registers with calls to ipa_reg_offset() or ipa_reg_n_offset().

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>