net: ipa: metadata_mask register is RX only

The INIT_HDR_METADATA_MASK endpoint configuration register is only
valid for RX endpoints.  Rather than writing a zero to that register
for TX endpoints, avoid writing the register at all.

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