fm10k: reduce the scope of the q_idx local variable

Reduce the scope of the q_idx local variable in the fm10k_cache_ring_qos
function.

This was detected by cppcheck and resolves the following style warning
produced by that tool:

[fm10k_main.c:2016]: (style) The scope of the variable 'q_idx' can be
reduced.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>