rtlwifi/rtl8723be: Replace magic number by macro

For consistency with other drivers, replace a magic number by a macro.

Signed-off-by: Rickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
Reviewed-by: Peter Wu <peter@lekensteyn.nl>
Signed-off-by: John W. Linville <linville@tuxdriver.com>