sdhci: Remove class "virtual" methods

The SDHCIClass defines a series of class "methods".  However, no code
in the QEMU tree overrides these methods or even uses them outside of
sdhci.c.

Remove the virtual methods and replace them with direct calls to the
underlying functions.  This simplifies the process of extending the
sdhci code to support PCI devices (which have a different parent
class).

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
