usb: musb: hsdma: add wrapper for reading DMA count

Add a wrapper for reading the DMA count register, analogous
to the one for writing to this register.

Signed-off-by: Anand Gadiyar <gadiyar@ti.com>
Cc: Vikram Pandita <vikram.pandita@ti.com>
Signed-off-by: Felipe Balbi <felipe.balbi@nokia.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
Signed-off-by: Tony Lindgren <tony@atomide.com>