staging: rts_pstor: Fix a bug that a MMCPlus card can't be accessed

1. Don't switch bus if cmd14 timedout
2. Add a new group of return codes for mmc_test_switch_bus

Signed-off-by: wwang <wei_wang@realsil.com.cn>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>