net: ipa: introduce ipa_reg()

Create a new function that returns a register descriptor given its
ID.  Change ipa_reg_offset() and ipa_reg_n_offset() so they take a
register descriptor argument rather than an IPA pointer and register
ID.  Have them accept null pointers (and return an invalid 0 offset),
to avoid the need for excessive error checking.  (A warning is issued
whenever ipa_reg() returns 0).

Call ipa_reg() or ipa_reg_n() to look up information about the
register before calls to ipa_reg_offset() and ipa_reg_n_offset().
Delay looking up offsets until they're needed to read or write
registers.

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