net: ipa: assign proper packet context base

At the end of ipa_mem_setup() we write the local packet processing
context base register to tell it where the processing context memory
is.  But we are writing the wrong value.

The value written turns out to be the offset of the modem header
memory region (assigned earlier in the function).  Fix this bug.

Tested-by: Sujit Kautkar <sujitka@chromium.org>
Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>