eeprom: Pull out the I/O code

Pull out the code which does the I2C or SPI read/write, so that
the beefy ifdef around it is contained in a single function.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Simon Glass <sjg@chromium.org>
Cc: Tom Rini <trini@konsulko.com>
Cc: Heiko Schocher <hs@denx.de>
Reviewed-by: Heiko Schocher <hs@denx.de>