net: phy: bcm7xxx: introduce r_rc_cal_reset helper

This function performs a R/RC calibration reset and will start being
used by more than one function in the next patches, create a helper
function to factor code.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>