libertas: get SD8688 rx length with one CMD52

Usually, the 16-bit rx length is read from scratch pad registers
with two CMD52 transactions:
SD8385: 	IF_SDIO_SCRATCH_OLD (0x80fe/0x80ff)
SD8686/SD8688:	IF_SDIO_SCRATCH     (0x34/0x35)

Alternatively, SD8688 firmware offers an enhanced method for driver
to read an 8-bit rx length (in units) with a single CMD52:
IF_SDIO_RX_UNIT 0x43 is read one time after firmware is ready.
IF_SDIO_RX_LEN  0x42 is read every time when rx interrupt is received.

Signed-off-by: Bing Zhao <bzhao@marvell.com>
Acked-by: Dan Williams <dcbw@redhat.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>