radio-tavarua: Update the low power mode flag during FM off operation

The lower power mode flag is not updated after the interrupts
have been disabled during FM turn Off operation. This causes the
interrupts to not to be enabled during next FM turn On
operation and this results in GPIO not toggling for XFR operations.

Change-Id: I0f2199292b570f1add2bc23c9cf6bcd480781c03
CRs-Fixed: 344780
Signed-off-by: Anantha Krishnan <ananthk@codeaurora.org>