Rename sdhci_cam_start_slot() into sdhci_start_slot()

This change allows to just call sdhci_start_slot() in SDHCI drivers
and not to think about which stack handles the operation.

As a side effect, this will also fix MMCCAM with sdhci_acpi driver.

Approved by: imp (mentor)
Differential Revision: https://reviews.freebsd.org/D12471