net: ipa: make filter/routing hash enable register variable

For IPA v3.5.1, the IPA filter/routing hash enable register actually
does exist, but it is at offset 0x8c into the IPA register space.
For newer versions of IPA it is at offset 0x148.

Define a new inline function ipa_reg_filt_rout_hash_en_offset() to
return the appropriate value for a given version of IPA hardware.

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