net: ipa: kill ev_ch_e_cntxt_1_length_encode()

Now that we explicitly define each register field width there is no
need to have a special encoding function for the event ring length.
Add a field for this to the EV_CH_E_CNTXT_1 GSI register, and use it
in place of ev_ch_e_cntxt_1_length_encode() (which can be removed).

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