mmc: rtsx_usb_sdmmc: Remove set but unused variable 'err'

mmc: rtsx_usb_sdmmc: Remove set but unused variable 'err'

Seeing as sd_wait_data_idle() returns void and only a very specific
read value is checked, there doesn't seem to be a good reason to
check the return value of rtsx_pci_read_register().  So remove
the assignment and definition of 'err'.

Squashes the following W=1 kernel build warning:

 drivers/mmc/host/rtsx_usb_sdmmc.c: In function ‘sd_wait_data_idle’:
 drivers/mmc/host/rtsx_usb_sdmmc.c:657:6: warning: variable ‘err’ set but not used [-Wunused-but-set-variable]
 657 | int err, i;
 | ^~~

Cc: Roger Tseng <rogerable@realtek.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20200701124702.908713-11-lee.jones@linaro.org
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>