platform: msm: uim: fix return value of write operation

Uim driver returns the number of bytes written. It was incorrectly
returning 0 in all cases.

Change-Id: I2b443e6dad01f0996327a37f7dc9548f9eaecac1
Acked-by: Ho Lee <holee@qti.qualcomm.com>
Signed-off-by: Venkatesh Yadav Abbarapu <quicvenkat@codeaurora.org>
Signed-off-by: Gilad Avidov <gavidov@codeaurora.org>