USB: imx21-hcd: refactor hardware data memory management

We already have fields describing the hardware data memory
(dmem_size and dmem_offset) in the HCD private data,
use them rather than the rather obscure read from the
hardware descriptor.

Signed-off-by: Martin Fuzzey <mfuzzey@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>