battery: Correct battery_get_disconnect_state() return
battery: Correct battery_get_disconnect_state() return

This cannot return a value from a different enum. Doing so produces an
error with clang:

   battery_fuel_gauge.c:228:10: warning: implicit conversion from
	enumeration type 'enum ec_error_list' to different enumeration
	type 'enum battery_disconnect_state' [-Wenum-conversion]

Fix it by returning the existing error value.

BUG=none
BRANCH=none
TEST=CQ

Signed-off-by: Simon Glass <c00b0378376498bd9cd974c388df8854c0131d27@chromium.org>
Change-Id: I1f3826ef26b1255934ea96cd1541f60436546b57
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/2842204
Reviewed-by: Denis Brockus <6b21e8190fb5e373363fb95271367073a16474ef@chromium.org>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec