mtd: rawnand: gpmi: put only once used functions inline

read_page_prepare(), read_page_end() and read_page_swap_end() are
trivial functions that are used only once and take 8 arguments each.
De-obfuscate the code by open coding these functions in
gpmi_ecc_read_page()

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>