hwmon: (pmbus) Initialise page count in pmbus_identify()

The `pmbus_identify()` function fails to correctly determine the number
of supported pages on PMBus devices. This occurs because `info->pages`
is implicitly zero-initialised, and `pmbus_set_page()` does not perform
writes to the page register if `info->pages` is not yet initialised.
Without this patch, `info->pages` is always set to the maximum after
scanning.

This patch initialises `info->pages` to `PMBUS_PAGES` before the probing
loop, enabling `pmbus_set_page()` writes to make it out onto the bus
correctly identifying the number of pages. `PMBUS_PAGES` seemed like a
reasonable non-zero number because that's the current result of the
identification process.

Testing was done with a PMBus device in QEMU.

Signed-off-by: Titus Rwantare <titusr@google.com>
Fixes: 442aba7 ("hwmon: PMBus device driver")
Link: https://lore.kernel.org/r/20250227222455.2583468-1-titusr@google.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>

Author: Rwantare

drivers/hwmon/pmbus/pmbus.c

@@ -103,6 +103,8 @@ static int pmbus_identify(struct i2c_client *client,
 		if (pmbus_check_byte_register(client, 0, PMBUS_PAGE)) {
 			int page;
 
+			info->pages = PMBUS_PAGES;
+
 			for (page = 1; page < PMBUS_PAGES; page++) {
 				if (pmbus_set_page(client, page, 0xff) < 0)
 					break;