log: convert pr_*() to logging

In drivers we use a family of printing functions including pr_err() and
pr_cont(). CONFIG_LOGLEVEL is used to control which of these lead to output
via printf().

Our logging functions allow finer grained control of output. So replace
printf() by the matching logging functions. The usage of CONFIG_LOGLEVEL
remains unchanged.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>