staging: rtlwifi: refactor rtl_get_tcb_desc

Refactored rtl_get_tcb_desc slightly to stay within the 80 character
line limit.

Signed-off-by: Luke Triantafyllidis <ltriant@cpan.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>