drivers: mmc: Fix R2 response type definition

The Poplar is broken on eMMC initialization because of commit
2a82a9c9 ("drivers: emmc: dw_mmc: Add response flag into response ID
definition").  It changes the driver behavior on response type handling
in dw_send_cmd(), because MMC_RESPONSE_R(2) and MMC_RESPONSE_R2 are
different things.  MMC core is still sending the former while we already
changed to check the latter in dw_mmc driver.

This patch fixes R2 response type in MMC core code.  It's the same
thing as what commit 94522ff7

 ("drivers: mmc: Fix R3 response type
definition") does for R3 response.

With this fix, Poplar is back to work.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>