iwlwifi: bug fix in AGG flow - cast const to ULL

This patch fixes a bug in AGG flow:
u64 bitmap = 0;
bitmap |= 1 << 32 results to be 0xffffffff80000000.

Signed-off-by: Emmanuel Grumbach <emmanuel.grumbach@intel.com>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Zhu Yi <yi.zhu@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>