net: ipa: clean up header memory validation

Do some general cleanup in ipa_cmd_header_valid():
  - Delay assigning the mem variable until just before it's used.
  - Assign the maximum offset and size values together.
  - Improve comments explaining the single range of memory being
    made up of a modem portion and an AP portion.
  - Record the offset of the combined range in a local variable.
  - Do the initial size assignment right after assigning the offset.

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