mesh: Fix check for mkdir return value in keyring.c

mesh: Fix check for mkdir return value in keyring.c

Remove check for mkdir() return value, since checking for zero does not
cover "already exists" condition and adding extra check for errno value
unnecessarily complicates the code. If mkdir() fails due to any reason
than "already exists", the subsequent call to open() fails and the
error is detected and corectly processed by the code.

Change-Id: I72db1d9553ba9a8269d391f938fbb26abc5cdf50
Signed-off-by: Abhay Agarwal <ay.agarwal@samsung.com>