staging: rtl8723bs: hal: sdio_ops: Mark used 'err' as __maybe_unused and remove another

'err' is only used if debug is enabled.

Also remove 4 year old TODO.

Fixes the following W=1 kernel build warning(s):

 drivers/staging/rtl8723bs/hal/sdio_ops.c: In function ‘sdio_read32’:
 drivers/staging/rtl8723bs/hal/sdio_ops.c:170:6: warning: variable ‘err’ set but not used [-Wunused-but-set-variable]
 drivers/staging/rtl8723bs/hal/sdio_ops.c: In function ‘sdio_read_mem’:
 drivers/staging/rtl8723bs/hal/sdio_ops.c:381:6: warning: variable ‘err’ set but not used [-Wunused-but-set-variable]

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Fabio Aiuto <fabioaiuto83@gmail.com>
Cc: linux-staging@lists.linux.dev
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20210414181129.1628598-47-lee.jones@linaro.org


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