xrow: fix xrow_decode_synchro rejecting non-int types

There was an error in xrow_decode_synchro: it compared the expected type
of the value to the type of the key (MP_UINT) instead of the type of the
actual value. This went unnoticed because all values in synchro requests
were integers.

This is going to change soon, when PROMOTE requests will start holding a
vclock, so fix the wrong type check.

In-scope-of #9138

NO_DOC=bugfix
NO_CHANGELOG=not user-visible

(cherry picked from commit c18410f5)