regulator: refgen: avoid overwriting bandgap bypass control bit

Avoid overwriting bit 0 (BANDGAP_BYPASS) of the bandgap control
register when enabling and disabling a refgen regulator.  This
bit is configured by boot loaders to be either 0 or 1 based on
per-part requirements.

Change-Id: Idc4f2c746ae9de7ba3f5badcbdc3b55b9d765892
Signed-off-by: David Collins <collinsd@codeaurora.org>