staging: rts_pstor: set lun_mode in a different place

In sony notebook, card reader will be configured as two-lun mode
through EFUSE. In this situation, MS card can't be recognized
properly because driver does only support single-lun mode in default.
In this patch, lun_mode is set in a different place, so driver can
detect the EFUSE configuation automatically.

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