e1000: Delay safe_pause switch until SI_SUB_CLOCKS

Based on sysinit_sub_id, SI_SUB_CLOCKS is after SI_SUB_CONFIGURE.

SI_SUB_CONFIGURE  = 0x3800000,  /* Configure devices */  
At this stage, the variable “cold” will be set to 0.

SI_SUB_CLOCKS    = 0x4800000,  /* real-time and stat clocks*/
At this stage, the clock configuration will be done, and the real-time
clock can be used.

In the e1000 driver, if the API safe_pause_* are called between
SI_SUB_CONFIGURE and SI_SUB_CLOCKS stages, it will choose the wrong
clock source. The API safe_pause_* uses “cold” the value of which is
updated in SI_SUB_CONFIGURE, to decide if the real-time clock source is
ready. However, the real-time clock is not ready til the SI_SUB_CLOCKS
routines are done.

Obtained from: Juniper Networks
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D42920