net: ipa: HOL_BLOCK_EN_FMASK is a 1-bit mask

The convention throughout the IPA driver is to directly use
single-bit field mask values, rather than using (for example)
u32_encode_bits() to set or clear them.

Fix the one place that doesn't follow that convention, which sets
HOL_BLOCK_EN_FMASK in ipa_endpoint_init_hol_block_enable().

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