fix(megaraid): remove cache policy prefixes on logical volume creation

[ezekiel-alexrod]

What

Fix logical volume creation on MegaRAID controllers: cache policy tokens
were emitted with the set-command prefixes (rdpolicy=, wrcache=,
iopolicy=), which the add vd grammar does not accept.

Why

storcli2 add vd takes cache policies as bare positional tokens, not as
key=value pairs. Passing the prefixed form made every creation fail:

$ storcli64 /c0 add vd type=raid0 drives=251:1 rdpolicy=nora wrcache=wb iopolicy=direct
syntax error, unexpected TOKEN_UNKNOWN, expecting $end

The key=value form is only valid on the set path (e.g. rdcache=,
wrcache=), which is left untouched.

What changed

megaraidCreateCacheFlags now returns the bare tokens from the mappers
(nora, wb, direct, ...) instead of prefixing them. The generated
command becomes:

/c0 add vd type=raid0 drives=251:1 nora wb direct

Testing

• go build ./...
• go test ./... (all green; updated TestMegaraidCreateCacheFlags
  expectation to the bare-token form)