ixgbevf: fix unused variable warning

The new ixgbevf_set_rx_buffer_len() function causes a harmless warnings
in configurations with large page size:

drivers/net/ethernet/intel/ixgbevf/ixgbevf_main.c: In function 'ixgbevf_set_rx_buffer_len':
drivers/net/ethernet/intel/ixgbevf/ixgbevf_main.c:1758:15: error: unused variable 'max_frame' [-Werror=unused-variable]

This rephrases the code so that the compiler can see the use of that
variable, making it slightly easier to read in the process.

Fixes: f15c5ba5 ("ixgbevf: add support for using order 1 pages to receive large frames")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Acked-by: Alexander Duyck <alexander.h.duyck@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>