rtlwifi: rtl8192cu: fix function declaration header

Move rtl92cu_update_hal_rate_tbl declaration to hw.h to correspond with function
definition in hw.c.
Fixes sparse warning in hw.c warning: symbol 'rtl92cu_update_hal_rate_tbl' was
not declared.

Signed-off-by: Mark Schulte <schultetwin@gmail.com>
Acked-by: Larry Finger <Larry.Finger@lwfinger.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>