i40e: use explicit cast from u16 to u8

BugLink: http://bugs.launchpad.net/bugs/1536474

Current implementation generates compilation warnings.

Change-ID: Icceefb50fe62aefaf90a64afb7192e08355a4ec5
Signed-off-by: Kamil Krawczyk <kamil.krawczyk@intel.com>
Acked-by: Shannon Nelson <shannon.nelson@intel.com>
Tested-by: Andrew Bowers <andrewx.bowers@intel.com>
Signed-off-by: Jeff Kirsher <jeffrey.t.kirsher@intel.com>
(cherry picked from commit 6621e4b2)
Signed-off-by: Tim Gardner <tim.gardner@canonical.com>

drivers/net/ethernet/intel/i40e/i40e_lan_hmc.c

@@ -762,7 +762,7 @@ static void i40e_write_byte(u8 *hmc_bits,
 
 	/* prepare the bits and mask */
 	shift_width = ce_info->lsb % 8;
-	mask = BIT(ce_info->width) - 1;
+	mask = (u8)(BIT(ce_info->width) - 1);
 
 	src_byte = *from;
 	src_byte &= mask;