IB/iser: Inline two work request conversion functions

Since the next patch will change the return type of these functions into a
const pointer and since the iSER driver modifies the work request these
functions return a pointer two, inline two work request conversion
function calls. This patch does not change any functionality.

Signed-off-by: Bart Van Assche <bart.vanassche@wdc.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Max Gurtovoy <maxg@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>