rtlwifi: rtl8192c-common: rtl8192se: rtl8192de: Simplify if statements

Devendra Naga <devendra.aaru@gmail.com> submitted a patch for rtl8192c_common
to change the tests in _rtl92c_store_pwrIndex_diffrate_offset(). This patch
improves on those changes and applies similar modifications to drivers rtl8192se
and rtl8192de.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>