board: icore-rqs: Fix mmc get env device

As per USDHC boot eFUSE descriptions:
USDHC3 => devno 2
USDHC4 => devno 3

Linux will detect mmc0, mmc1, mmc2 based on the status
"okay" on usdhc so imx6qdl-icore-rqs.dtsi has enabled
usdhc1, usdhc3 and usdhc4.But U-Boot can detect based
on the aliases so add mmc1, mmc2 for usdhc3 and usdhc4
respectively and return the board_mmc_get_env_dev
by subtracting -1

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>