ixgbevf: add spinlocks for MTU change calls

Protect set_rlpml with mailbox lock to make sure the MTU configuration
is handled properly.

This change resolves an issue where set_rlpml can fail when the VF
interface is brought up:
ixgbevf 0000:03:1d.6: Failed to set MTU at 1500

Signed-off-by: Emil Tantilov <emil.s.tantilov@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>