net: ipa: don't use u32p_replace_bits()

In two spots we use u32_replace_bits() to replace a set of bits in a
register while preserving the rest.  Both of those cases just zero
the bits being replaced, and this can be done more simply without
using that function.

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