iwlwifi: set TX_CMD_FLAG_PROT_REQUIRE_MSK in tx_flag

When building tx command, always set TX_CMD_FLAG_PROT_REQUIRE_MSK
for 5000 series and up.

Without setting this bit the firmware will not examine the RTS/CTS setting
and thus not send traffic with the appropriate protection. RTS/CTS is is
required for HT traffic in a noisy environment where, without this setting,
connections will stall on some hardware as documented in the patch that
initially attempted to address this:

    commit 1152dcc2
    Author: Wey-Yi Guy <wey-yi.w.guy@intel.com>
    Date:   Fri Jan 15 13:42:58 2010 -0800

    iwlwifi: Fix throughput stall issue in HT mode for 5000

    Similar to 6000 and 1000 series, RTS/CTS is the recommended
    protection mechanism for 5000 series in HT mode based on the HW design.
    Using RTS/CTS will better protect the inner exchange from interference,
    especially in highly-congested environment, it also prevent uCode encounter
    TX FIFO underrun and other HT mode related performance issues.

For 3945 and 4965, different flags are used for RTS/CTS or CTS-to-Self
protection.

Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>