staging: mt7621-mmc: Fix ret.cooci warnings

Return the value directly instead of storing it in local variable err.
Remove the unused local variable err.
The semantic patch that fixes the first part of the problem is as follows

// <smpl>
@@
local idexpression ret;
expression e;
@@

-ret =
+return
     e;
-return ret;
// </smpl>

Signed-off-by: Bhanusree Pola <bhanusreemahesh@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>