fix initial conditions for EC SEI model

[valentinsulzer]

Description

Small bug fix

[codecov]

Codecov Report

Merging #1895 (6788c46) into develop (c8a01ba) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop    #1895   +/-   ##
========================================
  Coverage    99.28%   99.28%           
========================================
  Files          345      345           
  Lines        18901    18901           
========================================
  Hits         18765    18765           
  Misses         136      136           

Impacted Files	Coverage Δ
...ybamm/models/submodels/interface/sei/sei_growth.py	100.00% <100.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update c8a01ba...6788c46. Read the comment docs.