Fix usage of FF_createERR macro in prvGetFAT12Entry (#62)

Description
-----------
#45 introduced a compilation error about implicit `FF_CreateError`
function when using FAT12 support. I assume it was supposed to be
`FF_createERR` macro instead so I changed it.

Checklist:
----------
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ff_fat.c

@@ -372,7 +372,7 @@ FF_Buffer_t * prvGetFromFATBuffers( FF_IOManager_t * pxIOManager,
 
         if( FF_isERR( xError ) )
         {
-            xError = FF_CreateError( FF_GETERROR( xError ), FF_GETFATENTRY );
+            xError = FF_createERR( FF_GETERROR( xError ), FF_GETFATENTRY );
         }
         else
         {