staging: rtl8723bs: remove unnecessary extern in os_dep/sdio_intf.c

remove unnecessary extern.

The function is defined static in os_dep/os_intfs.c and used only once
in the same file

Signed-off-by: Fabio Aiuto <fabioaiuto83@gmail.com>
Link: https://lore.kernel.org/r/f3a7b2c00221aa09ed782fb30ce55936f443a12d.1616503354.git.fabioaiuto83@gmail.com


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