staging: rtl8822be: Keep array subscript no lower than zero

The kbuild test robot reports the following:
   drivers/staging//rtlwifi/phydm/phydm_dig.c: In function 'odm_pause_dig':
   drivers/staging//rtlwifi/phydm/phydm_dig.c:494:45: warning: array subscript is below array bounds [-Warray-bounds]
      odm_write_dig(dm, dig_tab->pause_dig_value[max_level]);

This condition is caused when a loop falls through. The fix is to pin
max_level to be >= 0.

Signed-off-by: Larry Finger <Larry.Finger@lwfinger.net>
c: kbuild test robot <fengguang.wu@intel.com>
Fixes: 9ce99b04 staging: r8822be: Add phydm mini driver
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>