samus: increase extpower task stack size
samus: increase extpower task stack size

Increase extpower task stack size due to occasional stack
overflow in extpower task on plugging/unpluggin AC.

BUG=chrome-os-partner:36760
BRANCH=samus
TEST=load onto samus, connect/disconnect a bunch of times
and check high water mark on stack usage: 368/512

Change-Id: I4531176ce6f15356a2267cdecf907a7694567da9
Signed-off-by: Alec Berg <1a4429eeda29095a12e19a73405f80f5b7f37a63@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/249920
Reviewed-by: Shawn N <5259333ce42e3ace6a73988cec7115ed9d3a661a@chromium.org>

md5555/ec,eatbyte/chromium-ec,eatbyte/chromium-ec,coreboot/chrome-ec,akappy7/ChromeOS_EC_LED_Diagnostics,coreboot/chrome-ec,md5555/ec,coreboot/chrome-ec,fourier49/BIZ_EC,fourier49/BIZ_EC,md5555/ec,akappy7/ChromeOS_EC_LED_Diagnostics,coreboot/chrome-ec,fourier49/BIZ_EC,md5555/ec,akappy7/ChromeOS_EC_LED_Diagnostics,fourier49/BZ_DEV_EC,fourier49/BIZ_EC,coreboot/chrome-ec,eatbyte/chromium-ec,fourier49/BZ_DEV_EC,akappy7/ChromeOS_EC_LED_Diagnostics,akappy7/ChromeOS_EC_LED_Diagnostics,fourier49/BZ_DEV_EC,coreboot/chrome-ec,eatbyte/chromium-ec,fourier49/BZ_DEV_EC