spi/rtc-{ds1390,ds3234,m41t94}: Use spi_get_drvdata() for SPI devices

The recently increased type checking in platform_get_drvdata() reveals a few
offenders:

drivers/rtc/rtc-ds1390.c:161: warning: passing argument 1 of ‘platform_get_drvdata’ from incompatible pointer type
drivers/rtc/rtc-ds3234.c:161: warning: passing argument 1 of ‘platform_get_drvdata’ from incompatible pointer type
drivers/rtc/rtc-m41t94.c:139: warning: passing argument 1 of ‘platform_get_drvdata’ from incompatible pointer type

Use spi_get_drvdata() instead of platform_get_drvdata().

Signed-off-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>