fm10k: reduce scope of *p local variable

fm10k: reduce scope of *p local variable

Reduce the scope of the char *p local variable to only the block where
it is used.

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

[fm10k_ethtool.c:229]: (style) The scope of the variable 'p' 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>