rtlwifi: rtl8192ce: rtl8192cu: Fix endian issue

Commit e996db69 (rtlwifi: rtl8192c: Add init codes for "fw_version"
and "fw_subversion") added initialization for fw_version, but failed to
note that the variable in the firmware header is little-endian. The
following Sparse warning results:

  CHECK   drivers/net/wireless/rtlwifi/rtl8192c/fw_common.c
drivers/net/wireless/rtlwifi/rtl8192c/fw_common.c:242:36: warning: incorrect type in assignment (different base types)
drivers/net/wireless/rtlwifi/rtl8192c/fw_common.c:242:36:    expected unsigned short [unsigned] [usertype] fw_version
drivers/net/wireless/rtlwifi/rtl8192c/fw_common.c:242:36:    got restricted __le16 [usertype] version

When fixing this problem, I noticed that several of the drivers contain
nearly identical copies of the firmware header struct, and that only the one
used in rtl8192c{e,u} had correct endian notation. The struct has been
moved into a common header, and the other drivers will be fixed in
subsequant patches.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>