staging: rtl8723bs: Fix camel-case in IS_FW_HEADER_EXIST_8723B().

Change the parameter of the macro to the snake case 'fw_hdr' instead
of '_pFwHdr'.

Signed-off-by: Quytelda Kahja <quytelda@tamalin.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>