Fix some warnings in n770_defconfig build:
drivers/mtd/nand/omap-hw.c:390: warning:
'omap_nand_write_byte' defined but not used

tahvo-usb.c, tahvo-user.c and retu-user.c: ignoring return
value of 'xxx', declared with attribute warn_unused_result

Signed-off-by: Dirk Behme <dirk.behme_at_gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>