rt2x00: rt2800: convert read_eeprom functions to return an int value

Both the rtt2x00usb_eeprom_read and the ioremap
functions are allowed to fail, however their
return values are not checked in the read_eeprom
functions in the rt2800{pci,usb} drivers.

The patch adds the missing checks, and converts
all read_eeprom functions to return an int value,
so the error values can be propagated up to the
'rt2800_validate_eeprom' function.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: John W. Linville <linville@tuxdriver.com>