spi25_statusreg: make register read/write functions generic

This patch adds new spi_{read,write}_register() functions that take the
source/destination register as an argument. Currently they can only
access SR1, support for other registers will be added in another patch.

Since we're refactoring things, this commit also makes
spi_read_register() return an error code, making it possible to identify
error conditions that spi_read_status_register() concealed.

This also removes the initial 100ms delay between writing a register and
the first attempt to check the chip's status. An initial delay was added
to avoid needing to read the status register multiple times, but that is
unlikely to cause problems on modern flash chips.

BUG=b:195381327,b:153800563
BRANCH=none
TEST=flashrom -{r,w,E}
TEST=flashrom --wp-{enable,disable,range,list,status} at end of patch series

Change-Id: I0a3951bbf993f2d8d830143b29d3ce16cc6901d7
Signed-off-by: Nikolai Artemiev <nartemiev@google.com>
Reviewed-on: https://review.coreboot.org/c/flashrom/+/58475
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Nico Huber <nico.h@gmx.de>
Reviewed-by: Anastasia Klimchuk <aklm@chromium.org>
Reviewed-by: Edward O'Callaghan <quasisec@chromium.org>

flash.h

@@ -166,6 +166,13 @@ struct flashrom_flashctx;
 #define flashctx flashrom_flashctx /* TODO: Agree on a name and convert all occurences. */
 typedef int (erasefunc_t)(struct flashctx *flash, unsigned int addr, unsigned int blocklen);
 
+enum flash_reg {
+	INVALID_REG = 0,
+	STATUS1,
+	STATUS2,
+	MAX_REGISTERS
+};
+
 struct flashchip {
 	const char *vendor;
 	const char *name;