arm: using -EAGAIN directly instead of _k_neg_eagain

_k_neg_eagain is there for used in assembly where including
errno.h is not possible. However, the usage in ARM was simply
to assign value to swap_return_value in a C file, which is
no need to use _k_neg_eagain as errno.h can be included.
So change that to use -EAGAIN directly. Saves 4 bytes in
built binaries in rodata.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>