staging:rtl8192u: drop explicit externs

This patch removes use of explicit extern specifier and quietens the
sparse warnings:
drivers/staging/rtl8192u/r8192U_dm.c:159:1: warning: function 'init_hal_dm' with external linkage has definition
drivers/staging/rtl8192u/r8192U_dm.c:179:13: warning: function 'deinit_hal_dm' with external linkage has definition
drivers/staging/rtl8192u/r8192U_dm.c:245:17: warning: function 'hal_dm_watchdog' with external linkage has definition
drivers/staging/rtl8192u/r8192U_dm.c:278:13: warning: function 'init_rate_adaptive' with external linkage has definition
drivers/staging/rtl8192u/r8192U_dm.c:878:17: warning: function 'dm_txpower_trackingcallback' with external linkage has definition
drivers/staging/rtl8192u/r8192U_dm.c:1609:13: warning: function 'dm_cck_txpower_adjust' with external linkage has definition
drivers/staging/rtl8192u/r8192U_dm.c:1776:13: warning: function 'dm_change_dynamic_initgain_thresh' with external linkage has definition
drivers/staging/rtl8192u/r8192U_dm.c:1846:1: warning: function 'dm_change_fsync_setting' with external linkage has definition
drivers/staging/rtl8192u/r8192U_dm.c:1863:1: warning: function 'dm_change_rxpath_selection_setting' with external linkage has definition
drivers/staging/rtl8192u/r8192U_dm.c:2543:13: warning: function 'dm_init_edca_turbo' with external linkage has definition
drivers/staging/rtl8192u/r8192U_dm.c:2663:13: warning: function 'DM_CTSToSelfSetting' with external linkage has definition
drivers/staging/rtl8192u/r8192U_dm.c:2782:17: warning: function 'dm_rf_pathcheck_workitemcallback' with external linkage has definition
drivers/staging/rtl8192u/r8192U_dm.c:3142:13: warning: function 'dm_fsync_timer_callback' with external linkage has definition
drivers/staging/rtl8192u/r8192U_dm.c:3481:13: warning: function 'dm_shadow_init' with external linkage has definition

Signed-off-by: Himangi Saraogi <himangi774@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>