mmc: sdhci-esdhc-imx: refactor clock loopback selection logic

mmc: sdhci-esdhc-imx: refactor clock loopback selection logic

i.MX reference manual specifies that internal clock loopback should be
used for SDR104/HS200/HS400 modes. Move ESDHC_MIX_CTRL_FBCLK_SEL
configuration into the timing selection function to:

1. Explicitly set internal loopback path for SDR104/HS200/HS400 modes
2. Avoid redundant bit manipulation across multiple functions

Preserve ESDHC_MIX_CTRL_FBCLK_SEL during system resume for SDIO devices
with MMC_PM_KEEP_POWER and MMC_PM_WAKE_SDIO_IRQ flag, as the controller
might lose register state during suspend while skipping card
re-initialization.

Signed-off-by: Luke Wang <ziniu.wang_1@nxp.com>
Acked-by: Adrian Hunter <adrian.hunter@intel.com>
Link: https://lore.kernel.org/r/20250521025502.112030-1-ziniu.wang_1@nxp.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>