fm10k: use spinlock to implement mailbox lock

Lets not re-invent the locking wheel. Remove our bitlock and use
a proper spinlock instead.

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>