staging: most: change dma_buf variable to __le16

dma_buf is being cast to __le16 *, but it was defined as u16 *.
sparse reported this error as:
drivers/staging/most/hdm-usb/hdm_usb.c:158:16: warning: cast to restricted
__le16

This patch changes dma_buf from u16 to __le16.

Signed-off-by: Ramiro Oliveira <Ramiro.Oliveira@synopsys.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>