OvmfPkg/MemEncryptSevLib: clean up MemEncryptSevSetPageEncMask() decl
The declaration and the definition(s) of the function should have
identical leading comments and/or identical parameter lists. Replace any
leftover "clear" references to the C-bit with "set" references. Also
remove any excess space in the comment block, and unindent the trailing
"**/" if necessary. Correct several parameter references.