Platform/RPi3: Use Wait4Bit return value consistently

There are multiple points in the existing code that take the UINT32
value returned by Wait4Bit and return it directly as an EFI_STATUS
value.  If Wait4Bit fails, this will result in a spurious EFI_STATUS
value of 0x1 (EFI_WARN_UNKNOWN_GLYPH) being returned to the caller.  A
caller treating any non-zero value as an error will see this as a
failure, but a caller using the EFI_ERROR macro will incorrectly see
it as a success.

Fix by defining the return type of Wait4Bit as EFI_STATUS and ensuring
that a valid EFI_STATUS value is always returned.

Signed-off-by: Pete Batard <pete@akeo.ie>
Reviewed-by: Leif Lindholm <leif.lindholm@linaro.org>