mmc: renesas_sdhi: Add missing checks for the presence of quirks

mmc: renesas_sdhi: Add missing checks for the presence of quirks

When running on an system without any quirks (e.g. R-Car V3U), the
kernel crashes with a NULL pointer dereference:

    Unable to handle kernel NULL pointer dereference at virtual address 0000000000000002
    ...
    Hardware name: Renesas Falcon CPU and Breakout boards based on r8a779a0 (DT)
    Workqueue: events_freezable mmc_rescan
    ...
    Call trace:
     renesas_sdhi_internal_dmac_start_dma+0x54/0x12c
     tmio_process_mrq+0x124/0x274

Fix this by adding the missing checks for the validatity of the
priv->quirks pointer.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Tested-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/cc3178c2ff60f640f4d5a071d51f6b0b1db37656.1648822020.git.geert+renesas@glider.be
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>