net: ipa: don't reuse variable names

In ipa_endpoint_init_hdr(), as well as ipa_endpoint_init_hdr_ext(),
a top-level automatic variable named "offset" is used to represent
the offset of a register.

However, deeper within each of those functions is *another*
definition of a local variable with the same name, representing
something else.  Scoping rules ensure the result is what was
intended, but this variable name reuse is bad practice and makes
the code confusing.

Fix this by naming the inner variable "off".  Use "off" instead of
"checksum_offset" in ipa_endpoint_init_cfg() for consistency.

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