NVMe: Simplify Firmware Activate code slightly

Add definitions for the three Firmware Activate actions, and change the
SCSI translation code to construct the command into a temporary variable
instead of translating the endianness back-and-forth.

Signed-off-by: Matthew Wilcox <matthew.r.wilcox@intel.com>
Reviewed-by: Vishal Verma <vishal.l.verma@linux.intel.com>