drivers: sdhci-brcmstb: work around mystery CQE CMD_IDLE_TIMER trampling

For unknown reasons the controller seems to reset the idle polling timer
interval on CQE enable/disable to 8 clocks which is extremely short.

Just use the reset value in the eMMC spec (4096 clock periods which at
200MHz is ~20uS).

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