firmware: ti_sci: Include linux/err.h in ti_sci_protocol.h

The common TI SCI header file uses some macros from err.h and these
get exercised when CONFIG_TI_SCI_PROTOCOL is not defined. Include
the linux/err.h header file in this header file directly rather
than relying on source files to include it to eliminate any
potential build errors.

While at this, reorder the existing header file include to the
beginning of the file.

Signed-off-by: Suman Anna <s-anna@ti.com>
Reviewed-by: Nishanth Menon <nm@ti.com>