net: ipa: share field mask values for IPA hash registers

The IPA filter/routing hash enable register and filter/routing hash
flush register each have four single-bit fields representing the
four hashed tables to be enabled or flushed.  The field positions
are identical, so just use a single set of field masks to represent
the fields for both registers.

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