mmc: brcmstb: don't squash card-busy detection on bcm2712

Commit 485d9421  ("mmc: sdhci-brcmstb: check R1_STATUS for
erase/trim/discard") introduced a new flag and defaulted to disabling
card busy detection across all platforms with this controller.

This is required for IO voltage switching, as the card drives CMD low
while the switch is in progress.

Signed-off-by: Jonathan Bell <jonathan@raspberrypi.com>