spcom: avoid implicit casting from size_t to uint32 for file ops

spcom expose "pseudo file" char-device to user space.
The file operation callback function definition is using size_t for user
buffer size.
The spcom API uses uint32 as the spcom communication buffers are about
300 bytes, and the user space file operation read()/write() are limited to
PAGE_SIZE which is about 4KB.
Use explicit casting rather than implicit casting when fops callbacks
calls spcom internal functions.

CRs-Fixed: 2025174
Change-Id: I5d6bf71ab77f97ea350178bb8b3473fd4cefddfd
Signed-off-by: Amir Samuelov <amirs@codeaurora.org>