md: dm-req-crypt: Reduce the max hw_sectors for dm-req-crypt queue

Split IO requests for dm-req-crypt to improve encryption performance
of smaller file sizes.

Change-Id: I4e0a6f011e92accf7f797d4a37b0640185cbe175
Acked-by: Baranidharan Muthukumaran <bmuthuku@qti.qualcomm.com>
Signed-off-by: Dinesh K Garg <dineshg@codeaurora.org>