Unbreak build when SPIFFS_CACHE==0.

[jmattsson]

• This PR is for the dev branch rather than for master.
• This PR is compliant with the other contributing guidelines as well (if not, please describe why).
• I have thoroughly tested my contribution.
• The code changes are reflected in the documentation at docs/en/*.

It seems at some point we lost the ability to build if SPIFFS_CACHE is defined to be 0 in user_config.h. The spiffs code bails out due to SPIFFS_CACHE_WR not being defined. This fixes that problem by providing a default definition for it.

[jmattsson]

I acknowledge we (DiUS) seem to be the only ones using the filesystem as read-only, so it's probably not worth pushing this issue upstream.

[pjsg]

Looks plausible. But I would merge it in -- it might save somebody else.