IB/qib: Don't rely on (undefined) order of function parameter evaluation

Some of the qib sysfs code passes a buffer pointer into 
simple_read_from_buffer() but relies on a function call in another 
parameter of the same call to initialize that pointer.  Since the order
of evaluation of function parameters is undefined, this will break if
gcc chooses the wrong order.

Fix this by splitting the code into two separate function calls.

This was noticed because of warnings like the following on ppc:

    drivers/infiniband/hw/qib/qib_fs.c: In function 'portcntrs_2_read':
    drivers/infiniband/hw/qib/qib_fs.c:203: warning: 'counters' is used uninitialized in this function

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Roland Dreier <rolandd@cisco.com>