eeprom: Pull out transfer length computation

Pull out the code which computes the length of the transfer
into separate code and clean it up a little. 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>