wifi: rtlwifi: use helper function rtl_get_hdr()

Although rtl_get_hdr was added in commit 3dad618b ("rtlwifi: Change
wifi.h for rtl8192se and rtl8192de"), it is currently not being utilized
in some areas. This commit replaces the open code with the rtl_get_hdr()
function.

Signed-off-by: Juhee Kang <claudiajkang@gmail.com>
Reviewed-by: Simon Horman <simon.horman@corigine.com>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230527114954.3281-1-claudiajkang@gmail.com