mmc: rtsx_pci: Enable MMC_CAP_ERASE to allow erase/discard/trim requests

Cc: Micky Ching <micky_ching@realsil.com.cn>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>
Tested-by: Mauro Santos <registo.mailling@gmail.com>