drivers: char: add stream like read support

Some user space clients with stream like reading may read
partial data from skb. Currently only reading of entire skb
is supported.

Adding support to read smaller chunks from an entire skb.

Change-Id: I7789bea78debab8ece517a99a0066eccbf9a64ec
Signed-off-by: Sarannya S <sarannya@codeaurora.org>
Signed-off-by: Deepak Kumar Singh <deesin@codeaurora.org>
Signed-off-by: shouyi Hu <shouhu@codeaurora.org>