fix(buttons): Fix an I2C transaction without bus lock

The call to MAX77650_getDebounceStatusnEN0() in epic_buttons_read()
performs transactions on the I2C bus but is not guarded by locking the
I2C hwlock.  This leads to strange issues like USB mode deadlocking when
writing large files.

Widen the hwlock I2C section to encompass the MAX77650_getDebounceStatusnEN0()
call as well.  This fixes #189.

Signed-off-by: Rahix <rahix@rahix.de>