misc: fastrpc: fix error code in fastrpc_req_mmap()

The "err" variable shadows an earlier global variable so this code
returns success instead of a negative error code.

Fixes: 532ad70c ("misc: fastrpc: Add mmap request assigning for static PD pool")
Signed-off-by: Dan Carpenter <error27@gmail.com>
Link: https://lore.kernel.org/r/Y4XUrMETCl1O6t0A@kili


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>