net: ipa: head-of-line block registers are RX only

The INIT_HOL_BLOCK_EN and INIT_HOL_BLOCK_TIMER endpoint registers
are only valid for RX endpoints.

Have ipa_endpoint_modem_hol_block_clear_all() skip writing these
registers for TX endpoints.

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