USB: storage: ums-realtek: fix error code in rts51x_read_mem()

USB: storage: ums-realtek: fix error code in rts51x_read_mem()

The rts51x_read_mem() function should return negative error codes.
Currently if the kmalloc() fails it returns USB_STOR_TRANSPORT_ERROR (3)
which is treated as success by the callers.

Fixes: 065e60964e29 ("ums_realtek: do not use stack memory for DMA")
Acked-by: Alan Stern <stern@rowland.harvard.edu>
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/20220304073504.GA26464@kili
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>