fix(ufs): set the PRDT length field properly

The PRDT length field contains the count of the entries in the PRDT. See
JEDEC Standard No. 223E, section 6.1.1, "UTP Transfer Request
Descriptor," page 66. Previously we were setting the PRDT length field
to the number of bytes in the PRDT divided by four (the size in units of
32 bits). This was incorrect according to the spec.

Signed-off-by: Jorge Troncoso <jatron@google.com>
Change-Id: I960771e6ce57002872392993042fae9ec505447e