sf: stmicro: drop redundant id read

The common SPI flash code reads the idcode and passes it down to the SPI
flash driver, so there is no need to read it again ourselves.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Acked-by: Haavard Skinnemoen <haavard.skinnemoen@atmel.com>
CC: Jason McMullan <mcmullan@netapp.com>
CC: TsiChung Liew <Tsi-Chung.Liew@freescale.com>