aio: drop needless assignment in aio_read()

Commit 84c4e1f8 ("aio: simplify - and fix - fget/fput for io_submit()")
refactored aio_read() and some error cases into early return, which made
some intermediate assignment of the return variable needless.

Drop this needless assignment in aio_read().

No functional change. No change in resulting object code.

Signed-off-by: Lukas Bulwahn <lukas.bulwahn@gmail.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>