net: ipa: avoid setting an undefined field

The GSI channel protocol field in the CH_C_CNTXT_0 GSI register is
widened starting IPA v5.0, making the CHTYPE_PROTOCOL_MSB field
added in IPA v4.5 unnecessary.  Update the code to reflect this.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>