staging: rtl8188eu: move another static array from .h to .c

Move the declaration of the rtw_cmd_callback array from rtw_cmd.h to
rtw_cmd.c.

The _RTW_CMD_C_ symbol is now obsolete and can be removed.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20210408195601.4762-8-martin@kaiser.cx


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>