net/tftp: sparse fixes

tftp.c:464:17: warning: cast to restricted __be16
tftp.c:552:29: warning: cast to restricted __be16
tftp.c:640:33: warning: cast to restricted __be16
tftp.c:642:25: warning: cast to restricted __be16

Signed-off-by: Kim Phillips <kim.phillips@freescale.com>
Cc: Joe Hershberger <joe.hershberger@gmail.com>