rtl8192c: fix compilation errors

On my G5 this fails to compile with

drivers/net/wireless/rtlwifi/rtl8192c/phy_common.c:701: error: __ksymtab__rtl92c_phy_txpwr_idx_to_dbm causes a section type conflict
drivers/net/wireless/rtlwifi/rtl8192c/phy_common.c:701: error: __ksymtab__rtl92c_phy_txpwr_idx_to_dbm causes a section type conflict
drivers/net/wireless/rtlwifi/rtl8192c/phy_common.c:677: error: __ksymtab__rtl92c_phy_dbm_to_txpwr_Idx causes a section type conflict
drivers/net/wireless/rtlwifi/rtl8192c/phy_common.c:677: error: __ksymtab__rtl92c_phy_dbm_to_txpwr_Idx causes a section type conflict

since you can't export static functions.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>