media: sp8870: coding style fixes: miscellaneous

media: sp8870: coding style fixes: miscellaneous

This patch fixes the following checkpatch:

ERROR:ASSIGN_IN_IF: do not use assignment in if condition
WARNING:BRACES: braces {} are not necessary for any arm of this statement
WARNING:CONST_STRUCT: struct dvb_frontend_ops should normally be const
WARNING:EXPORT_SYMBOL: EXPORT_SYMBOL(foo); should immediately follow its function/variable
CHECK:ALLOC_SIZEOF_STRUCT: Prefer kzalloc(sizeof(*state)...) over kzalloc(sizeof(struct sp8870_state)...)
CHECK:AVOID_EXTERNS: extern prototypes should be avoided in .h files
CHECK:COMPARISON_TO_NULL: Comparison to NULL could be written ...
CHECK:MULTIPLE_ASSIGNMENTS: multiple assignments should be avoided

Signed-off-by: Stefan Herdler <herdler@nurfuerspam.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>