arm: semihosting: get rid of forward declarations

arm: semihosting: get rid of forward declarations

By rearranging the functions in the semihosting code we can
avoid forward-declaration of the internal static functions.
This puts the stuff in a logical order: read/open/close/len
and then higher-order functions follow at the end.

Cc: Darwin Rambo <[email protected]>
Cc: AKASHI Takahiro <[email protected]>
Cc: Mark Hambleton <[email protected]>
Cc: Tom Rini <[email protected]>
Acked-by: Steve Rae <[email protected]>
Signed-off-by: Linus Walleij <[email protected]>