mmc: am654_sdhci: Use MMC_MODES_END value instead of hardcoded value

The hardcoded array size leads to array overflows with changes in
speed modes enum in mmc core. Use MMC_MODES_END for otap_del_sel
array declaration to fix this.

Signed-off-by: Faiz Abbas <faiz_abbas@ti.com>
Reviewed-by: Peng Fan <peng.fan@nxp.com>