eeprom: Pull out the RW loop

Unify the code for doing read/write into single function, since the
code for both the read and write is almost identical. This again
trims down the code duplication.

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>