fm10k: use variadic arguments to fm10k_add_stat_strings

Instead of using a fixed prefix string we setup before each call to
fm10k_add_stat_strings, modify the helper to take variadic arguments and
pass them to vsnprintf. This requires changing the fm10k_stat strings to
take % format specifiers where necessary, but the resulting code is much
simpler.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Tested-by: Krishneil Singh <krishneil.k.singh@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>