net: rmnet_data: Fix assignments, reads, and logic

Global variables are no longer assigned values. Instead
flush meta data is handled and tracked at per end
point basis.

Reduce the number of register reads for timer. Leverage
value which has already been read earlier in function.

Change-Id: If0f3362aff1f92c94d05367c4b6428b897ff6ede
Signed-off-by: Conner Huff <chuff@codeaurora.org>