Staging: rts5208: rtsx_card.c: Remove unused function

The functions double_depth, check_card_fail, check_card_ejected are not
used anywhere in the kernel. So, remove their prototype and definition.
Grepped to find occurences.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>