sdhci: don't infer a non-removable slot on Apollo Lake SDXC

Intel Apollo Lake SDXC controller reports a Slot Type of
"Embedded Slot for One Device" in SDHCI_CAPABILITIES bits, even
when the slot is a removable card reader.
This caused 48 timeouts before the boot sequence resumed.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D58467