spi/rtc-m41t93: Use spi_get_drvdata() for SPI devices

One new offender detected by the recently increased type checking in
platform_get_drvdata():

drivers/rtc/rtc-m41t93.c: In function ‘m41t93_remove’:
drivers/rtc/rtc-m41t93.c:192: warning: passing argument 1 of ‘platform_get_drvdata’ from incompatible pointer type

Use spi_get_drvdata() instead of platform_get_drvdata(), cfr. commit
42fea15d ("spi/rtc-{ds1390,ds3234,m41t94}:
Use spi_get_drvdata() for SPI devices")

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