spl: remove redundant prints in boot_from_devices

spl: remove redundant prints in boot_from_devices

The null check for loader in boot_from_devices was moved earlier in the
code path by the commit ae409a84e7bff ("spl: NULL check variable before
dereference"), therefore the subsequent null checks for loader are not
necessary.

This patch removes those checks and refactors the prints to be more
useful in case of errors.

Signed-off-by: Anshul Dalal <anshuld@ti.com>