fm10k: reduce the scope of the tx_buffer variable

The tx_buffer local variable in the function fm10k_clean_tx_ring is not
used except inside a smaller block scope. Reduce the scope to its point
of use.

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

[fm10k_netdev.c:179]: (style) The scope of the variable 'tx_buffer' 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>